docs(P3a): inline images for self-contained HTML (REQ-268)

New scripts/inline_images.py (stdlib only: base64, re, mimetypes) —
base64-embeds all relative-path <img src='assets/...'> images into
the rendered HTML so it's redistributable without the assets/ folder.
MIME-sniffs by extension (.png->image/png, .svg->image/svg+xml, etc).
render_slides.sh Step 3 invokes it after the MARP HTML render, before
staging. Verified: 2 images inlined, 0 file-path refs remaining.

---ci---
project: acdl
phase: 3
milestone: v1.23
status: execute
phase_role: execution
---/ci---

scripts/inline_images.py

@@ -0,0 +1,72 @@
+#!/usr/bin/env python3
+"""scripts/inline_images.py — base64-embed all relative-path images in an
+HTML file so it becomes self-contained (redistributable without the
+assets/ folder).
+
+Usage: python scripts/inline_images.py <html-path>
+
+Stdlib only (base64, re, mimetypes, sys, pathlib).
+"""
+from __future__ import annotations
+
+import base64
+import mimetypes
+import re
+import sys
+from pathlib import Path
+
+IMG_SRC_RE = re.compile(
+    r'(<img\b[^>]*\bsrc=")(assets/[^"]+)("[^>]*>)',
+    re.IGNORECASE,
+)
+
+
+def _mime_for(path: Path) -> str:
+    ext = path.suffix.lower()
+    if ext == ".svg":
+        return "image/svg+xml"
+    guessed, _ = mimetypes.guess_type(str(path))
+    return guessed or "application/octet-stream"
+
+
+def inline(html_path: Path) -> int:
+    html = html_path.read_text(encoding="utf-8")
+    repo_root = html_path.parent.parent.parent
+
+    count = 0
+
+    def replacer(match: re.Match[str]) -> str:
+        nonlocal count
+        prefix, rel_src, suffix = match.group(1), match.group(2), match.group(3)
+        img_path = html_path.parent / rel_src
+        if not img_path.exists():
+            print(f"  WARNING: image not found: {rel_src}", file=sys.stderr)
+            return match.group(0)
+        mime = _mime_for(img_path)
+        data = base64.b64encode(img_path.read_bytes()).decode("ascii")
+        count += 1
+        return f'{prefix}data:{mime};base64,{data}{suffix}'
+
+    new_html = IMG_SRC_RE.sub(replacer, html)
+
+    if count > 0:
+        html_path.write_text(new_html, encoding="utf-8")
+
+    return count
+
+
+def main() -> int:
+    if len(sys.argv) != 2:
+        print("Usage: python scripts/inline_images.py <html-path>", file=sys.stderr)
+        return 1
+    html_path = Path(sys.argv[1])
+    if not html_path.exists():
+        print(f"ERROR: {html_path} not found", file=sys.stderr)
+        return 1
+    count = inline(html_path)
+    print(f"Inlined {count} image(s) into {html_path}", file=sys.stderr)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())