[rush-daemon] Add request scheduler foundation

[Mo Jazayeri (mojaza)]

Summary

Issue #5897 requires daemon requests to queue instead of failing when another Rush command is active. This PR introduces the initial @rushstack/rush-daemon package and its request admission scheduler as an independently reviewable foundation for that workstream.

Details

The scheduler defines SHARED-BUILD, SHARED-READ, and EXCLUSIVE compatibility classes. Requests in the same shared class can run concurrently, incompatible classes serialize, and a queued exclusive request gates later admissions to preserve FIFO fairness.

Admission supports queue position notifications, clean cancellation through AbortSignal, bounded waits, and fail-fast --no-wait semantics. Leases are idempotently released so a completed request cannot corrupt scheduler state.

This is intentionally a partial implementation of #5897: transport, workspace session routing, command classification, graph integration, and shared-build selection merging depend on the WS0/WS1 prerequisites and are not included here. The package is registered but not connected to the Rush CLI, so existing behavior is unchanged.

How it was tested

• Built @rushstack/rush-daemon with Rush/Heft, including TypeScript, ESLint, and API Extractor.
• Ran 7 scheduler unit tests covering shared concurrency, incompatible serialization, exclusive FIFO gating, fail-fast admission, timeout, cancellation, and queue position updates.

Impacted documentation

N/A. The package is not yet exposed through the Rush CLI.

[Sean Larkin (TheLarkInn)]

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