Fix server resolution, module responses, and HTTP caching - #1403

Fix server correctness issues that caused package resolution to hang, required initialization imports to disappear, and valid module requests to fail.

• Correct package and export resolution, including GitHub version ranges and backup-registry retries.
• Install missing CSS packages, generate valid CommonJS default-export wrappers, and separate transform caches by filename while preserving /tsx URLs.
• Honor compression preferences, cache variation and configured TTLs; preserve custom landing-page status and modification checks.
• Add regression tests for all 14 findings.

Validation: go test ./server, focused race-detector checks, and seven Deno integration tests covering transforms, CSS modules, scoped forks and export filtering all passed locally.