feat: Add KeyedFairSemaphore

[tisonkun]

Summary

• add mea::keyed_fair_semaphore::KeyedFairSemaphore with borrowed and owned permits
• schedule queued waiters by the fewest in-flight permits for their key, preserving queue-order ties
• cover keyed fairness, per-key FIFO, cancellation, capacity stress, docs, and Send/Sync/Unpin assertions

Verification

• cargo x test
• cargo +nightly x lint

[Copilot]

Pull request overview

This PR introduces a new synchronization primitive, mea::keyed_fair_semaphore::KeyedFairSemaphore, to bound global concurrency while distributing permits fairly across independent “keys” (e.g., tenants/partitions) based on lowest in-flight counts with FIFO tie-breaking.

Changes:

• Added the keyed_fair_semaphore module implementing KeyedFairSemaphore plus borrowed/owned permit types.
• Added a dedicated test suite covering fairness behavior, FIFO within a key, cancellation, and capacity stress.
• Updated crate docs/README to list and link the new primitive, and added Send/Sync/Unpin assertions in lib.rs tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
README.md	Documents the new KeyedFairSemaphore and links to its docs.rs page.
mea/src/lib.rs	Exposes the new module and adds trait assertions for the new types.
mea/src/keyed_fair_semaphore/mod.rs	Implements the keyed fair semaphore, permits, and scheduling logic.
mea/src/keyed_fair_semaphore/tests.rs	Adds unit/stress tests for fairness, cancellation, and capacity invariants.

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