fix(condvar): align notifications with standard semantics

[tisonkun]

Summary

• replace the semaphore-backed Condvar state with a fair FIFO waiter queue
• make notify_one and notify_all affect only tasks that are currently waiting
• register each waiter before releasing its mutex guard to preserve the atomic wait handoff
• pass a notify_one wakeup to another current waiter when the selected wait is cancelled before reacquiring the mutex
• document predicate-based usage and cover non-buffering, one/all wakeups, cancellation, and owned waits

Testing

• cargo x lint
• cargo x test --no-capture
• cargo test --workspace --all-targets
• cargo +1.85.0 test --workspace --no-default-features

[Copilot]

Pull request overview

This PR reworks Condvar to follow standard condition-variable semantics (non-buffered notifications) while preserving fairness via a FIFO waiter queue, and adds documentation/tests to lock in the new behavior.

Changes:

• Replace the semaphore-backed Condvar state with an internal FIFO waiter queue and explicit waiter state tracking.
• Update the internal WaitList API to support unlink-then-remove semantics and adapt Semaphore to the new API.
• Add targeted Condvar tests for non-buffered notifications, notify-one/all behavior, and cancellation handoff; update README/CHANGELOG to reflect the new semantics.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates Condvar description to match FIFO + non-buffered semantics.
mea/src/internal/waitlist.rs	Refactors wait list into a sentinel-based list with explicit unlink/remove operations.
mea/src/internal/semaphore.rs	Migrates semaphore waiter handling to the new WaitList::unlink_* / remove_unlinked_waiter API.
mea/src/condvar/tests.rs	Adds coverage for standard condvar notification semantics and cancellation behavior.
mea/src/condvar/mod.rs	Implements the new FIFO, non-buffered Condvar with cancellation-aware notify-one baton passing.
CHANGELOG.md	Notes the user-visible semantic change to Condvar notifications and cancellation behavior.

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