Back ManagedCriticalState with Synchronization.Mutex

[lhoward]

Description

ManagedCriticalState is a ManagedBuffer around an os_unfair_lock, which makes Sources/Supporting/ManagedCriticalState.swift the only platform-specific code in Sources/. This replaces it with a Synchronization.Mutex held in a final-class box, so the wrapper keeps the reference semantics and copyability that the iterators and state machines rely on, and the API (withCriticalRegion, apply(criticalState:), criticalState) is unchanged. The file goes from 45 to 34 lines and the package then builds on Linux as well.

The cost is the deployment target: Mutex needs macOS 15 / iOS 18 / tvOS 18 / watchOS 11 / visionOS 2, so Package.swift moves to that floor (tools 5.9 only for the visionOS entry; the language mode is unchanged). If you would rather keep the current floor, a #if canImport(Darwin)-gated variant that leaves the os_unfair_lock path byte-for-byte and uses Mutex only elsewhere is available instead.

Tests

Linux, Swift 6.4, x86_64, with the two test-only fixes from the companion PR applied (an unused import Combine and a sync test that returns before its task; without them the test target does not compile here), three consecutive runs:

Executed 199 tests, with 0 failures (0 unexpected) in 15.396 (15.396) seconds
Executed 199 tests, with 0 failures (0 unexpected) in 15.653 (15.653) seconds
Executed 199 tests, with 0 failures (0 unexpected) in 15.802 (15.802) seconds

Checklist

• this PR is based on the main branch and is up-to-date
• the commits inside this PR have explicit commit messages
• unit tests cover the new feature or the bug fix (no behaviour change; the existing suite covers the lock)
• the CHANGELOG is up-to-date (happy to add an entry if you want this)

🤖 Generated with Claude Code

https://claude.ai/code/session_01T8cahEd4GfsHQ89sNrmP4p

[twittemb]

Closing as superseded by merged PR #74: #74.

Current main now uses Synchronization.Mutex in a reference-backed ManagedCriticalState, preserves shared state between iterator copies and callbacks, raises the supported Apple deployment targets, and requires Swift 6.1+. Linux release builds and the full test suite pass in CI on Swift 6.1.3 and 6.4.0. This covers the Linux compatibility change proposed here.

The additional explicit visionOS 2 platform declaration in this PR was not included in #74; that can be considered separately as a visionOS support follow-up.