Ship Material Symbols in the 20 optical-size cut

M3 draws icons at 20 dp from the optical size 20 cut, whose strokes stay as
heavy as the 24 dp design's instead of thinning with the glyph (styles.md
§ Icons, the optical size axis). The package only shipped the 24 cut, so every
20 px icon was a scaled-down 24 (core audit C16). bin/fetch-symbols now checks
out both cuts from the same commit of google/material-design-icons and writes
the new one to resources/svg/symbols/{outlined-20,filled-20}: 4,135 symbols
each, the same names and the same normalisation as the 24 cut.

Plan step 10; finding C16.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/svg/symbols/outlined-20/youtube_video.svg

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