fix: the favicon and nav brand mark rendered blank — "--" inside an XML comment

7ca6ee0's favicon.svg had a comment naming the `--fc-chrome-rgb` custom property. `--` is illegal inside an XML comment, so the file is not well-formed XML, and a browser renders an SVG-as-image only if it parses. It fails without any error, just a blank image. Because TopNav's brand glyph is the same file, the tab icon and the nav mark both went missing on the #251 deploy. logo.svg parses and was unaffected.

The comment now names the property without the hyphens, and says why it has to.

tests/test_public_svgs.py parses every SVG under frontend/public. It includes a vacuity guard (the directory really contains the two known files) and a positive control (the exact defect raises ParseError), so the check can actually fail.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

frontend/public/favicon.svg

@@ -15,7 +15,10 @@
        Colours are theme tokens (frontend/src/theme/fabled-tokens.js): obsidian
        plate, accent gold. The plate is kept here (unlike logo.svg) so the tab
        icon is self-contained against any browser chrome; on the nav it is
-       invisible because it matches --fc-chrome-rgb exactly. -->
+       invisible because it matches the fc-chrome-rgb custom property exactly.
+       No double hyphen may appear inside this comment: XML forbids it, and a
+       browser refuses to render an SVG that does not parse (it happened once —
+       tests/test_public_svgs.py). -->
   <rect width="32" height="32" rx="6" fill="#14171A"/>
   <rect x="6.2" y="4.2" width="19.6" height="23.6" rx="1.4"
         fill="none" stroke="#A87338" stroke-width="2.4"/>

tests/test_public_svgs.py

@@ -0,0 +1,40 @@
+"""Every SVG the browser loads from `frontend/public/` must parse as XML.
+
+A browser renders an SVG used as an image only if it is well-formed XML, and
+when it is not, nothing reports it: no console error in most browsers, no
+failed build, no failed request — the icon is simply blank. `favicon.svg`
+shipped that way (merge #251) because its comment contained a CSS custom
+property name, and `--` is illegal inside an XML comment. Both the tab icon and
+the nav brand mark went missing, and only a person looking at the page noticed.
+"""
+from __future__ import annotations
+
+import xml.etree.ElementTree as ET
+from pathlib import Path
+
+import pytest
+
+PUBLIC = Path(__file__).resolve().parent.parent / "frontend" / "public"
+SVGS = sorted(PUBLIC.rglob("*.svg"))
+
+
+def test_the_public_dir_has_svgs_to_check():
+    """Guards the guard: a moved directory would otherwise pass vacuously."""
+    assert {p.name for p in SVGS} >= {"favicon.svg", "logo.svg"}
+
+
+@pytest.mark.parametrize("svg", SVGS, ids=lambda p: p.name)
+def test_svg_is_well_formed_xml(svg):
+    root = ET.parse(svg).getroot()
+    assert root.tag == "{http://www.w3.org/2000/svg}svg"
+
+
+def test_the_parser_rejects_the_shape_that_broke_the_favicon():
+    """Positive control: the exact defect must fail this parser, or the
+    parametrized test above proves nothing."""
+    broken = (
+        '<svg xmlns="http://www.w3.org/2000/svg">'
+        "<!-- matches --fc-chrome-rgb exactly --></svg>"
+    )
+    with pytest.raises(ET.ParseError):
+        ET.fromstring(broken)