Docs: use a 3D-rendered plinth mark

Replaces the raster favicons with renders of the plinth pedestal, and puts a
rendered PLINTH wordmark at the top of the front page.

The POV-Ray sources live in _logo/ (underscore-prefixed, so Jekyll leaves them
out of the built site) and share a single include, so the wordmark and the icon
cannot drift apart. Palette is taken from the site's own CSS.

The flat artwork stays in use where rendering does not help: the 24px header
logo and the Safari pinned tab, both of which need to stay crisp and
single-colour. assets/images/favicon-source.svg remains the geometry the 3D
mark was matched against.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: hsyl20

_includes/svg/logo.svg

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+# Logo sources
+
+POV-Ray scenes for the 3D Plinth marks. Underscore-prefixed, so Jekyll does not
+copy any of it into the built site.
+
+| File | What |
+|---|---|
+| `plinth-common.inc` | palette, geometry, materials, light rig -- shared, so the two marks cannot drift apart |
+| `plinth-logo.pov` | the PLINTH wordmark |
+| `plinth-icon.pov` | the plinth alone on a brand tile, for the favicons |
+| `make-icons.py` | rounds the tile corners and cuts the favicon sizes |
+
+Rendering, from this directory:
+
+```sh
+# wordmark -> assets/images/plinth-wordmark.png (downscaled to 1200px wide)
+povray +Iplinth-logo.pov +Oplinth-logo.png +W1800 +H700 +A0.25 +AM2 +R3 +Q11
+
+# favicons -> assets/
+povray +Iplinth-icon.pov +Oplinth-icon.png +W1024 +H1024 +A0.2 +AM2 +R4 +Q11
+./make-icons.py plinth-icon.png
+```
+
+`Declare=Theme=1` renders either scene white-on-charcoal instead of
+coral-on-white. The site is light-skinned, so only the light variants are
+committed.
+
+The palette comes from the site's own CSS (`#fc4d50`, `#ffffff`, `#f2f2f2`,
+`#222222`). Two things in `plinth-common.inc` are worth reading before changing
+the lighting: `Note [sRGB hex under assumed_gamma 1.0]` and
+`Note [Hitting the brand colour]` -- the coral albedo is hand-calibrated against
+this light rig, and moving the lights invalidates it.
+
+The flat artwork these were matched against is still in the repo and still in
+use: `assets/images/favicon-source.svg` (geometry), `_includes/svg/logo.svg`
+(24px header mark) and `assets/safari-pinned-tab.svg` (monochrome mask).

_logo/README.md

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+#!/usr/bin/env python3
+"""Cut the favicon set from the square render of plinth-icon.pov.
+
+POV-Ray renders a square tile with hard corners; this rounds them off with an
+alpha mask and resamples down to the sizes the site uses. Corner radius is 22%
+of the edge, which is the iOS proportion and close to the existing flat mark.
+
+    povray +Iplinth-icon.pov +Oplinth-icon.png +W1024 +H1024 +A0.2 +AM2 +R4 +Q11
+    ./make-icons.py plinth-icon.png
+
+Outputs, named to drop straight into the site's assets directory:
+    apple-touch-icon.png         180
+    favicon-32x32.png             32
+    favicon-16x16.png             16
+    android-chrome-192x192.png   192
+    android-chrome-512x512.png   512
+    favicon.ico                   16, 32, 48
+    <stem>-rounded.png          full size, for READMEs and slides
+"""
+
+import sys
+from pathlib import Path
+
+from PIL import Image, ImageDraw
+
+# 12/64, the corner radius of the flat master artwork the site already ships in
+# assets/images/favicon-source.svg, so the 3D tile lines up with the header logo.
+RADIUS_FRACTION = 12 / 64
+SUPERSAMPLE = 4  # the mask is drawn large and shrunk, to antialias the corners
+PNG_SIZES = {
+    "apple-touch-icon.png": 180,
+    "favicon-32x32.png": 32,
+    "favicon-16x16.png": 16,
+    "android-chrome-192x192.png": 192,
+    "android-chrome-512x512.png": 512,
+}
+ICO_SIZES = [(16, 16), (32, 32), (48, 48)]
+
+
+def rounded_mask(size: int) -> Image.Image:
+    big = size * SUPERSAMPLE
+    mask = Image.new("L", (big, big), 0)
+    ImageDraw.Draw(mask).rounded_rectangle(
+        (0, 0, big - 1, big - 1), radius=int(big * RADIUS_FRACTION), fill=255
+    )
+    return mask.resize((size, size), Image.LANCZOS)
+
+
+def main(argv: list[str]) -> int:
+    src = Path(argv[1] if len(argv) > 1 else "plinth-icon.png")
+    prefix = argv[2] if len(argv) > 2 else ""
+
+    tile = Image.open(src).convert("RGBA")
+    if tile.width != tile.height:
+        print(f"{src}: expected a square render, got {tile.size}", file=sys.stderr)
+        return 1
+    tile.putalpha(rounded_mask(tile.width))
+
+    written = []
+    full = src.with_name(f"{src.stem}-rounded.png")
+    tile.save(full)
+    written.append((full.name, tile.width))
+
+    for name, size in PNG_SIZES.items():
+        out = src.with_name(prefix + name)
+        tile.resize((size, size), Image.LANCZOS).save(out, optimize=True)
+        written.append((out.name, size))
+
+    ico = src.with_name(prefix + "favicon.ico")
+    tile.save(ico, sizes=ICO_SIZES)
+    written.append((ico.name, ", ".join(str(w) for w, _ in ICO_SIZES)))
+
+    for name, size in written:
+        print(f"  {name:<24} {size}")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main(sys.argv))