Simplify build queue scheduling and shared results

[ije]

Concurrent requests for the same build could race with completion, and path normalization could remove another task from the queue. Replace the scheduler goroutine and per-request result channels with a FIFO pending queue and one shared completion signal per requested path.

Request cancellation now detaches the waiter while the shared build continues. Queue keys remain fixed, status updates are atomic, and joined requests receive the resolved build path. Concurrency limits, build timeouts, and panic recovery are preserved.

Pin the import-map resolution test to React DOM 19.2.4 so its scheduler 0.27.x expectation stays consistent with the fixture, and report setup errors directly.

Validation:

• go test ./server ./internal/... -count=1 passes.
• go test -race ./server -run '^TestBuildQueue' -count=25 passes.
• Regression coverage includes FIFO scheduling, deduplication, waiter cancellation, path changes, snapshots, failures, and retries.