[dombrovsky]

Synchronous event producers need to reserve capacity before deciding whether to accept an event or consume a cooldown. BoundedAdmissionController.TrySubmit returns an immediate Accepted / Full / Closed decision and a completion task on acceptance.

Closes #37.

API and behavior

• 12 overloads: keyed/unkeyed typed-state ValueTask cores and common Task conveniences, with optional cancellation and result support.
• Reuses existing queues and atomic global/per-key accounting. Preserves FIFO, eligible-waiter priority, round-robin scheduling, concurrency limits, cancellation, and lifetime ownership.
• One guarded drain dispatches accepted work off the producer's stack, including middleware and custom terminal scheduling callbacks. No capacity waits or admission waiters.
• Keeps Enqueue, Submit(Wait), and Submit(Reject). The latter supports enqueue middleware that cannot guarantee a synchronous decision. ITaskScheduler and terminal schedulers are unchanged.

Supported pipelines

Native pipelines support bounded/coordinated policies, annotations, and execution/completion middleware. Other enqueue middleware, opaque terminals, and multiple uncoordinated budgets fail before work creation or reservation.

Existing thread-affine context semantics remain. Immediate legacy submissions avoid unnecessary context capture; deferred work captures context so mixed producers can share the drain safely.

Documentation and validation

Usage guides contain API comparisons, examples, and compatibility rules; drain invariants live in the existing design notes. The temporary dispatch benchmark/profiling project is removed. Its harness and investigation remain available in commit history. Production dispatch regression tests remain.

• Release builds, analyzers, API baselines, formatting, and NativeAOT checks passed.
• Full implementation suite: 5,024/5,024 on .NET 8/10; C# 10/12 consumer checks passed.
• Documentation cleanup: examples compiled and ran on .NET 8/10, links/anchors checked, and 66/66 dispatch regression tests passed.