#161 Fix docs nav icon shading

Summary

• remove the custom sidebar SVG background override that painted gray blocks behind nav chevrons
• add a small regression test so sidebar SVG background rules do not come back

Tests

• node --test scripts/*.test.mjs
• node scripts/sweep-artifacts.mjs --check

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Summary by cubic

Fixes gray shading behind docs sidebar icons by removing the custom SVG background rules that painted blocks behind chevrons. Adds a regression test to ensure sidebar SVGs never set a background again.