fix: add cache-bust query param to Vite asset URLs for immutable caching

[bartlomieju]

Summary

• Vite-built assets (JS chunks, CSS) were served with Cache-Control: no-cache, no-store because the static files middleware only recognized builder-path assets (under /_fresh/js/{BUILD_ID}/) as immutable
• Adds an immutable flag to FileSnapshot, StaticFile, and PendingStaticFile interfaces
• The Vite plugin marks manifest chunks and CSS as immutable: true (these have content-hashed filenames), while public directory files remain non-immutable
• The static files middleware checks file.immutable alongside the existing path and query param checks

Closes #3282

Test plan

• All 10 static files unit tests pass
• All 31 Vite build integration tests pass (including new cache headers test)
• All 22 builder tests pass
• New test: vite build - asset cache headers on CSS and JS — fetches CSS and JS asset URLs from a page with islands and verifies they return Cache-Control: public, max-age=31536000, immutable

🤖 Generated with Claude Code

[bartlomieju]

Reviewed the diff — this looks good to merge:

• immutable flag is opt-in on the file interfaces, no breaking changes
• Only content-hashed Vite assets get marked, public dir files correctly excluded
• Static files middleware check is additive alongside existing BUILD_ID logic
• Bonus: hash passthrough fix in registerStaticFile and Windows path separator fix in server_snapshot.ts
• Good test — extracts real CSS/JS asset URLs from rendered HTML and verifies Cache-Control: public, max-age=31536000, immutable
• All CI green including Windows