docs(gallery): refresh canonical 38-type visual catalog (#119)

* docs(gallery): refresh canonical visual catalog

* docs: document screenshot renderer setup

scripts/screenshot_catalog.py

@@ -0,0 +1,54 @@
+"""Shared helpers for the canonical README screenshot catalog."""
+
+from __future__ import annotations
+
+import hashlib
+import re
+import struct
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parent.parent
+SKILL = ROOT / "skills/diagram-design/SKILL.md"
+ASSET_DIR = ROOT / "skills/diagram-design/assets"
+SCREENSHOT_DIR = ROOT / "docs/screenshots"
+MANIFEST = SCREENSHOT_DIR / "manifest.json"
+
+
+def canonical_slugs() -> list[str]:
+    """Return the visual-type order declared by SKILL.md's selection table."""
+
+    markdown = SKILL.read_text(encoding="utf-8")
+    start = markdown.find("### Visual-type guide")
+    end = markdown.find("Rules of thumb", start)
+    if start < 0 or end < 0:
+        raise ValueError("SKILL.md visual-type guide is missing")
+    return re.findall(
+        r"^\|[^\n]*\]\(references/type-([a-z0-9-]+)\.md\)\s*\|$",
+        markdown[start:end],
+        re.MULTILINE,
+    )
+
+
+def source_path(slug: str) -> Path:
+    return ASSET_DIR / f"example-{slug}.html"
+
+
+def screenshot_path(slug: str) -> Path:
+    return SCREENSHOT_DIR / f"{slug}.png"
+
+
+def sha256(path: Path) -> str:
+    digest = hashlib.sha256()
+    with path.open("rb") as handle:
+        for chunk in iter(lambda: handle.read(1024 * 1024), b""):
+            digest.update(chunk)
+    return digest.hexdigest()
+
+
+def png_dimensions(path: Path) -> tuple[int, int]:
+    with path.open("rb") as handle:
+        header = handle.read(24)
+    if len(header) != 24 or header[:8] != b"\x89PNG\r\n\x1a\n":
+        raise ValueError(f"not a PNG: {path}")
+    return struct.unpack(">II", header[16:24])