- Add comprehensive concurrency model documentation to serve.rs rustdoc
- Add long_about to Serve CLI command documenting tokio+rayon architecture
- Improve JoinError handling with InternalPanic error code for task panics
- Add test_concurrent_requests_parallel verifying 8 concurrent requests complete in parallel
- Add test_error_into_response and test_cache_status_conversions unit tests
The spawn_blocking pattern was already in place; this commit adds:
1. Documentation of the concurrency model in rustdoc and CLI help
2. Proper panic detection via JoinError::is_panic()
3. Error code INTERNAL_PANIC for panicking tasks
4. Integration test proving concurrent request parallelism
Closes: pdftract-jmh6w
