refactor: simplify test fixture and Rust config resolution
Test fixture:
- Reduce CSS files from 100 (5×20) to 15 (5×3), sufficient to test
per-directory PostCSS config resolution without excessive file count
- Deduplicate postcss-plugin.js into a single shared file under styles/
- Use array.map() pattern in Dir components instead of 20 manual elements
- Extract collectCss() helper in the test file
- Remove 360s timeouts (no longer needed with fewer files)
Rust:
- Refactor find_config_in_location() to express search order as a vec
of paths instead of scattered matches!/if-let blocks, making the
strategy for each PostCssConfigLocation variant explicit
Co-Authored-By: Claude <noreply@anthropic.com>
