feat(skirt-weights): flap snap + center tie seam + ride fade + top-band gates

Wrap skirt run approval (pairs with ariki-game c03c270d1):
- SKIRT_PIN_SNAP: each front vert glued to its OWN thigh (cross-leg blends
  park cloth between the legs at full split — the two-day pop-through bug).
- SKIRT_PIN_SEAM: ramp from own-thigh to 50/50 at the centerline — ties the
  two front flap ends without a hard weight step (a flat band read as a cut).
- SKIRT_RIDE_AZ_FADE: hard weight-zone boundaries animate as sawtooth.
- SKIRT_PIN_T_LO / SKIRT_RIDE_T_LO: keep the new belly rope pure pelvis.

clothing/skirt_garment_weights.py

@@ -127,6 +127,14 @@ PANTS_SOLID4 = os.environ.get("SKIRT_PANTS_SOLID4") == "1"
 # cloth stays strand-driven, where the thigh cage + groin spheres own the
 # interaction (that regime never holed). 0 (default) = pants all the way up.
 PANTS_FROM = _envf("SKIRT_PANTS_FROM", 0.0)
+# MIDLINE GATE (2026-08-22, wrap leg-springs architecture): when > 0, the pants
+# split applies ONLY inside ±<deg> of the forward axis — the front SECTORS
+# beyond it fall through to the classic strand+follow path, whose strands the
+# runtime re-roots onto the thighs (SKIRT_LEG_SPRINGS), so that cloth is carried
+# by leg springs instead of a rigid thigh split. The crotch midline keeps the
+# pack's pants answer; sides/back (|az| ≥ 80°) keep pants exactly as before.
+# 0 (default) = off, pants everywhere as before.
+PANTS_MIDLINE_AZ = _envf("SKIRT_PANTS_MIDLINE_AZ", 0.0)
 # FOLLOW TUBE (2026-08-20, Ozan: "the leg is a tube pushing in and out, not just the
 # line — govern the front quarter"): proximity follow keys off distance to the thigh
 # AXIS, so cloth beside the stitch line follows the line while the leg's VOLUME
@@ -173,6 +181,56 @@ ANCHOR_SECTOR_OUTER = _envf("SKIRT_ANCHOR_SECTOR_OUTER", 135.0)
 # > 0 blends each vertex across every strand within this many gaps using a
 # raised-cosine kernel, so folds curve instead of cornering. 0 = classic.
 AZ_SPREAD = _envf("SKIRT_AZ_SPREAD", 0.0)
+# WAISTBAND RIDE BAND (2026-08-22, wrap skirt — PokeDiag-measured): the slab
+# above the strand ring top is pelvis=1.00 by construction ("no skirt bone
+# reaches it"), and at ~90° hip flexion the advancing thigh's front surface
+# sweeps straight THROUGH that rigid cloth over each thigh front (worst vert
+# measured 9.5cm inside the skin, rest az ±45°). Horizontal ease can't help a
+# vert that never moves. SKIRT_RIDE_FRONT=<share> gives the pelvis-forced
+# verts in the front sector (|az off forward| in [RIDE_AZ_LO, RIDE_AZ_HI]) a
+# thigh split instead, so the rising thigh CARRIES that cloth over its front.
+# The midline and the back stay pelvis-rigid (a swinging center/back waistband
+# looks broken). 0 (default) = off, approved recipes bit-identical.
+RIDE_FRONT = _envf("SKIRT_RIDE_FRONT", 0.0)
+RIDE_AZ_LO = _envf("SKIRT_RIDE_AZ_LO", 30.0)
+RIDE_AZ_HI = _envf("SKIRT_RIDE_AZ_HI", 65.0)
+RIDE_AZ_FADE = _envf("SKIRT_RIDE_AZ_FADE", 12.0)  # waistband edge fade, degrees
+# Only apply the ride BELOW this drop fraction (2026-08-23, wrap rope): the
+# belly rope ring sits at the very top of the garment and must stay pure
+# pelvis — a thigh-riding rope ring distorts mid-stride. 0 (default) = whole
+# band rides (backwards compatible).
+RIDE_T_LO = _envf("SKIRT_RIDE_T_LO", 0.0)
+# HARD FRONT PIN (2026-08-22, wrap — PokeDiag: hem-tip verts at 84% thigh still
+# sink 3.7cm into the advancing thigh at 90° flexion; the leftover pelvis+strand
+# share is the drag, and the ANCHOR only redistributes the strand share, never
+# the pants/pelvis share). SKIRT_PIN_FRONT=<share> post-passes EVERY vert in the
+# front zone (az PIN_AZ_LO–HI off forward, drop fraction PIN_T_LO–HI) so its
+# thigh total is AT LEAST <share> — 1.0 = glued to the thigh surface, "the bone
+# that is already there" (Ozan: "can you just put the bone where the error
+# persists"). 0 (default) = off.
+PIN_FRONT = _envf("SKIRT_PIN_FRONT", 0.0)
+PIN_AZ_LO = _envf("SKIRT_PIN_AZ_LO", 20.0)
+PIN_AZ_HI = _envf("SKIRT_PIN_AZ_HI", 70.0)
+PIN_T_LO = _envf("SKIRT_PIN_T_LO", -0.4)
+PIN_T_HI = _envf("SKIRT_PIN_T_HI", 1.0)
+PIN_AZ_FADE = _envf("SKIRT_PIN_AZ_FADE", 12.0)  # edge fade width, degrees
+# FLAP SNAP (2026-08-22, run cycle): inside the pin zone the L/R split factor
+# f_l is a smooth ramp over hip width, so front-center verts land ~0.5/0.5
+# (or 0.7/0.3) across BOTH thighs — at full leg split those verts park between
+# the legs and the advancing thigh swallows them (PokeDiag run: pen -2.5cm at
+# az~180 hem point, w 0.68-0.88 mixed). SKIRT_PIN_SNAP=1 snaps f_l to the
+# vert's OWN side (±0.05 smooth transition at the centerline): the front hangs
+# as two flaps, each glued to its own thigh, like a real wrap's overlap.
+PIN_SNAP = _envf("SKIRT_PIN_SNAP", 0.0)
+# CENTER STRETCH SEAM (2026-08-22 v31, Ozan: "two skirt ends connect at the
+# center but don't cross the leg — stretch logic can be there"): with the pure
+# snap the front opens into a V down to the crotch at full leg split. Verts
+# within ±PIN_SEAM degrees of the front centerline override the snap with a
+# RAMP from the flap's own thigh (zone edge) to an exact 50/50 split
+# (centerline) — a narrow membrane that TIES the flaps together, stretches
+# between the legs, and parks at the midpoint BETWEEN them (never inside a
+# leg). 2026-08-23: flat 50/50 band → ramp (its 1.0→0.5 step read as a cut).
+PIN_SEAM = _envf("SKIRT_PIN_SEAM", 0.0)  # degrees each side of front center
 MAX_INFLUENCES = 4
 
 COMP_FMT = {5120: "b", 5121: "B", 5122: "h", 5123: "H", 5125: "I", 5126: "f"}
@@ -446,13 +504,85 @@ def main():
         f"cloth hangs {span:.3f} below the ring top")
 
     out_j, out_w = [], []
-    stats = {"waistband": 0, "follow_sum": 0.0, "anchored": 0}
+    stats = {"waistband": 0, "follow_sum": 0.0, "anchored": 0, "ride": 0, "pinned": 0}
     per_strand = {k: 0.0 for k in body_ring}
     thigh_total = 0.0
 
     for v in pos:
         w = {}
-        if v[1] > y_top - BAND:
+        # ── hard front pin (see the SKIRT_PIN_FRONT header): MUST run before the
+        # pants/SOLID4 branches — those pack and `continue`, so a post-pass after
+        # them never sees the hem verts (first attempt changed nothing, measured).
+        pinned = False
+        if PIN_FRONT > 0.0 and tube_fwd is not None:
+            fwd_ang = math.atan2(tube_fwd[1], tube_fwd[0])
+            rel = abs((math.atan2(v[2] - pelvis[2], v[0] - pelvis[0])
+                       - fwd_ang + math.pi) % (2 * math.pi) - math.pi)
+            t_rel = (y_top - v[1]) / span if span > 1e-9 else 0.0
+            # Edge-faded zone (Ozan 2026-08-22: "lower parts create triangle on
+            # the left while walking"): a HARD pin boundary tears a wedge where
+            # the pinned sector hugs the leg and the free midline hangs. Fade
+            # the share to 0 across PIN_AZ_FADE degrees on both sides instead.
+            fade = math.radians(PIN_AZ_FADE)
+            strength = (smoothstep((rel - (math.radians(PIN_AZ_LO) - fade)) / fade)
+                        * (1.0 - smoothstep((rel - math.radians(PIN_AZ_HI)) / fade)))
+            if strength > 0.0 and PIN_T_LO <= t_rel <= PIN_T_HI:
+                share = PIN_FRONT * strength
+                hip_half = max(0.02, abs(origin["thigh_l"][0] - pelvis[0]))
+                side = max(-1.0, min(1.0, (v[0] - pelvis[0]) / hip_half))
+                sign_l = 1.0 if origin["thigh_l"][0] >= pelvis[0] else -1.0
+                f_l = 0.5 + 0.5 * side * sign_l
+                if PIN_SNAP > 0.0:
+                    f_l = smoothstep((side * sign_l + 0.05) / 0.10)
+                if PIN_SEAM > 0.0 and rel < math.radians(PIN_SEAM):
+                    # TIE the flap ends (2026-08-23, Ozan: "a sharp cut, two
+                    # separate lines at full split — tie those end surfaces"):
+                    # a FLAT 50/50 seam band steps 1.0→0.5 at its boundary and
+                    # the step reads as a cut. Ramp f_l from the flap's own
+                    # thigh at the zone edge to 0.5 at the centerline instead —
+                    # the stretch distributes across the whole seam, no step.
+                    u = rel / math.radians(PIN_SEAM)
+                    f_own = 1.0 if (side * sign_l) >= 0.0 else 0.0
+                    f_l = 0.5 + (f_own - 0.5) * u
+                w[slot["pelvis"]] = 1.0 - share
+                w[slot["thigh_l"]] = share * f_l
+                w[slot["thigh_r"]] = share * (1.0 - f_l)
+                stats["pinned"] += 1
+                pinned = True
+        if pinned:
+            pass
+        elif v[1] > y_top - BAND:
+            # Waistband ride band (see the SKIRT_RIDE_FRONT header): front-sector
+            # waistband verts get a thigh split so the advancing thigh CARRIES the
+            # junction cloth over its front instead of sweeping through it.
+            ridden = False
+            t_rel_r = (y_top - v[1]) / span if span > 1e-9 else 0.0
+            if RIDE_FRONT > 0.0 and tube_fwd is not None and t_rel_r >= RIDE_T_LO:
+                fwd_ang = math.atan2(tube_fwd[1], tube_fwd[0])
+                rel = abs((math.atan2(v[2] - pelvis[2], v[0] - pelvis[0])
+                           - fwd_ang + math.pi) % (2 * math.pi) - math.pi)
+                fade_r = math.radians(RIDE_AZ_FADE)
+                if (math.radians(RIDE_AZ_LO) - fade_r <= rel
+                        <= math.radians(RIDE_AZ_HI) + fade_r):
+                    # Edge fade (2026-08-22 v31, Ozan: "too many triangles on
+                    # top skirt, looks distorted"): the hard 22°/65° boundary
+                    # put pelvis=1.0 verts next to pelvis=0.15 verts along the
+                    # waistband — a weight step that animates as a sawtooth top
+                    # edge. Fade the share over RIDE_AZ_FADE degrees both sides.
+                    r_str = (smoothstep((rel - (math.radians(RIDE_AZ_LO) - fade_r)) / fade_r)
+                             * (1.0 - smoothstep((rel - math.radians(RIDE_AZ_HI)) / fade_r)))
+                    if r_str > 0.0:
+                        ride = RIDE_FRONT * r_str
+                        hip_half = max(0.02, abs(origin["thigh_l"][0] - pelvis[0]))
+                        side = max(-1.0, min(1.0, (v[0] - pelvis[0]) / hip_half))
+                        sign_l = 1.0 if origin["thigh_l"][0] >= pelvis[0] else -1.0
+                        f_l = 0.5 + 0.5 * side * sign_l
+                        w[slot["pelvis"]] = 1.0 - ride
+                        w[slot["thigh_l"]] = ride * f_l
+                        w[slot["thigh_r"]] = ride * (1.0 - f_l)
+                        stats["ride"] += 1
+                        ridden = True
+            if not ridden:
                 w[slot["pelvis"]] = 1.0
             stats["waistband"] += 1
         else:
@@ -468,6 +598,13 @@ def main():
                 if PANTS_FROM > 0.0 and t_cloth < PANTS_FROM:
                     # fade pants IN across the same-width band below the line
                     pants_mix *= max(0.0, 1.0 - (PANTS_FROM - t_cloth) / blend_band)
+                if (PANTS_MIDLINE_AZ > 0.0 and tube_fwd is not None):
+                    fwd_ang = math.atan2(tube_fwd[1], tube_fwd[0])
+                    rel = abs((math.atan2(v[2] - pelvis[2], v[0] - pelvis[0])
+                               - fwd_ang + math.pi) % (2 * math.pi) - math.pi)
+                    deg = math.degrees(rel)
+                    if PANTS_MIDLINE_AZ < deg < 80.0:
+                        pants_mix = 0.0     # front sectors → strand/leg springs
             if pants_mix > 0.0:
                 hip_half = max(0.02, abs(origin["thigh_l"][0] - pelvis[0]))
                 side = max(-1.0, min(1.0, (v[0] - pelvis[0]) / hip_half))
@@ -668,6 +805,9 @@ def main():
     # ── gates FIRST: never leave a broken garment on disk ──
     n = len(pos)
     log(f"waistband (pelvis only): {stats['waistband']} verts")
+    if RIDE_FRONT > 0.0:
+        log(f"waistband ride band: {stats['ride']} verts thigh-split "
+            f"(share {RIDE_FRONT:.2f}, az {RIDE_AZ_LO:.0f}–{RIDE_AZ_HI:.0f}°)")
     log(f"mean thigh follow on panel verts: "
         f"{stats['follow_sum'] / max(1, n - stats['waistband']):.3f}")
     if ANCHOR: