Fix Vite CSS injection and Sass/node_modules handling

[fasterthanlime]

Summary

• fix Vite production CSS injection when script URLs are already rewritten to built assets
• add Sass load_paths plumbing so @use can resolve from node_modules
• include symlinked files when loading static/ and dist/
• add regressions for built-script CSS injection and symlinked static serving

Issues

• closes Vite production build snafu #239
• closes Vite: import SASS frameworks and fonts #240

Notes

• this currently allows filesystem fallback reads in the Sass cell for external imports
• follow-up for proper external dependency invalidation: Track external Sass dependencies for node_modules imports #242

Validation

• cargo nextest run -p cell-html
• cargo check -p cell-sass -p cell-sass-proto
• cargo xtask integration (currently failing in this environment; see run output in local command)