docs(humans): ship the MD explainer as a PDF with the images baked in

The HTML references screenshots relatively, so it only renders inside a
checkout -- mailing the file on its own gives broken images. The PDF is one
self-contained file, which is what actually gets sent to Ozlem.

- tools/humans_to_pdf.py: renders .humans/*.html via headless Chrome (or
  Edge), so the output is what the browser shows. HTML stays the source.
- @media print block on the page: the wide tables and the pipeline diagram
  scroll on screen but would be cut off at the paper edge; this reflows
  them and keeps figures, callouts and table rows off page breaks.
  Verified at A4 content width -- 6 pages, both screenshots embedded.
- *.pdf -> LFS. Each regeneration is a whole new ~2 MB blob, and unlike
  png/jpg there are no PDFs tracked raw yet, so there is no history to
  rewrite by adding it now.

Also corrects working-files.md: it claimed .gitattributes does not LFS-track
.blend, which was true when written and is no longer -- .blend/.zprj/.obj/
.npy/.npz have since been added. The note now covers what is actually still
excluded (png/jpg, deliberately) and why that makes QA renders scratch.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/humans_to_pdf.py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+"""Render a .humans/ HTML page to a self-contained PDF via headless Chrome.
+
+The pages in `.humans/` reference screenshots relatively
+(`../tools/tailor/screenshots/...`), so the HTML only works from inside a repo
+checkout. The PDF embeds those images, which makes it the thing you actually
+send to someone.
+
+The HTML stays the source of truth: edit the page, re-run this, commit both.
+
+    python tools/humans_to_pdf.py .humans/marvelous-designer.html
+    python tools/humans_to_pdf.py --all
+"""
+from __future__ import annotations
+
+import argparse
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+CHROME_CANDIDATES = [
+    r"C:\Program Files\Google\Chrome\Application\chrome.exe",
+    r"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe",
+    r"C:\Program Files\Microsoft\Edge\Application\msedge.exe",
+    r"C:\Program Files (x86)\Microsoft\Edge\Application\msedge.exe",
+    "/Applications/Google Chrome.app/Contents/MacOS/Google Chrome",
+    "/usr/bin/google-chrome",
+    "/usr/bin/chromium",
+]
+
+
+def find_chrome() -> Path:
+    for c in CHROME_CANDIDATES:
+        p = Path(c)
+        if p.exists():
+            return p
+    raise SystemExit("error: no Chrome or Edge found; tried:\n  "
+                     + "\n  ".join(CHROME_CANDIDATES))
+
+
+def to_pdf(html: Path, chrome: Path) -> Path:
+    out = html.with_suffix(".pdf")
+    # Chrome refuses to reuse a running profile, so give it a throwaway one.
+    with tempfile.TemporaryDirectory() as profile:
+        cmd = [
+            str(chrome),
+            "--headless=new",
+            "--disable-gpu",
+            f"--user-data-dir={profile}",
+            "--no-pdf-header-footer",       # drop the URL/date furniture
+            "--virtual-time-budget=15000",  # let local images decode first
+            f"--print-to-pdf={out}",
+            html.resolve().as_uri(),
+        ]
+        r = subprocess.run(cmd, capture_output=True, text=True, timeout=180)
+    if not out.exists():
+        sys.stderr.write(r.stderr or "")
+        raise SystemExit(f"error: Chrome produced no PDF for {html}")
+    return out
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description=__doc__,
+                                 formatter_class=argparse.RawDescriptionHelpFormatter)
+    ap.add_argument("page", type=Path, nargs="?", help="HTML page under .humans/")
+    ap.add_argument("--all", action="store_true", help="render every .humans/*.html")
+    args = ap.parse_args()
+
+    repo = Path(__file__).resolve().parent.parent
+    if args.all:
+        pages = sorted((repo / ".humans").glob("*.html"))
+    elif args.page:
+        pages = [args.page]
+    else:
+        ap.error("give a page, or --all")
+
+    if not pages:
+        print("nothing to render")
+        return 0
+
+    chrome = find_chrome()
+    print(f"using {chrome.name}")
+    for html in pages:
+        if not html.is_file():
+            print(f"  skip (missing): {html}")
+            continue
+        out = to_pdf(html, chrome)
+        print(f"  {out.stat().st_size / 1048576:.2f} MB  {out}")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())