feat(rasterize): SVG to PNG without Cairo, fonts pinned for ImageMagick

[andyed]

Why

The critique agent rasterized SVGs with cairosvg. On Windows, cairosvg installs and then fails at import because Windows has no libcairo-2.dll, so SVG audits there lose their visual pass.

What

python -m muriel.rasterize in.svg -o out.png (also muriel rasterize):

1. cairosvg first, where it imports. An OSError at import (installed, no Cairo) counts as unavailable, not as a crash.
2. ImageMagick 7 (magick) otherwise. Its Windows build carries librsvg and Cairo itself, so it needs nothing else installed.

The ImageMagick rung runs a font-pinning pass first, because its librsvg 2.40 has two problems with muriel's output:

• It tries only the first family of a font-family stack. muriel's generators lead with ui-sans-serif, -apple-system, system-ui, …, which never match, so every label fell back to a generic "Sans" with an expect ugly output warning. Stacks now resolve against magick -list font (attributes, inline styles, <style> sheets), and the CSS keywords map to the faces a browser picks (system-ui → Segoe UI, monospace → Consolas).
• It rounds intermediate weights to the nearest of 400 and 700, so the 600 on muriel's titles and axis labels rendered Bold. On Windows, each element that declares a font is rewritten to name the face directly ("Segoe UI Semibold" at weight 400), chosen by the CSS weight-matching rule. Weight pinning stands down when a <style> sheet sets fonts, or when the family lists only one face (a variable font).

The output reports the backend, each substitution with counts, any font fallbacks left in the render, and a note when the SVG has <image> elements. ImageMagick's librsvg drops or discolors those, so the note points to muriel capture (Chromium).

On boxes where cairosvg works, output is unchanged: pinning applies only on the ImageMagick rung.

Also updated: the critique agent now calls python -m muriel.rasterize instead of cairosvg, with matching changes to the svg channel doc, the squint docstring, the repo's .claude/settings.json allowlist, and CHANGELOG.

Test plan

• tests/test_rasterize.py: 40 tests. Pinning runs against a fixture font list, so it works on any OS. The ImageMagick render tests skip without magick.
• Full suite on Windows: 873 passed, 4 skipped.
• Rendered comparison-pair-serp.svg and comparison-pair-trace.svg on Windows. Unpinned, the headings fell back to a wide bold Sans (5 fallback warnings). Pinned, they render in Segoe UI Semibold and the mono cells in Consolas, with no warnings.
• Not exercised: the cairosvg rung on macOS or Linux (this box has no Cairo). That path only adds the OSError catch around the existing call.

🤖 Generated with Claude Code