Keep runtime schedulers safe through teardown

[matthargett]

Summary

• Share synchronized dispatch state so scheduler copies safely outlive their runtime.
• Close dispatch before AppRuntime discards pending work; retain finalizer cleanup and release dispatch captures outside the lock.
• Accept mutable and Napi::Env callbacks while preserving existing const/zero-argument overload behavior. Add ten bounded native tests with owned timeout state.

Validation

macOS 27.0 arm64, SDK 27.0, LLVM 23.1.1, real system JavaScriptCore. A native-only target compiled the committed scheduler test file and runtime, reusing dependency sources read-only:

• JSC_collectContinuously=1 SchedulerTests --gtest_repeat=20: 200/200 passed.
• Same run with ENABLE_THREAD_SANITIZER=ON, TSAN_OPTIONS=halt_on_error=1: 200/200 passed, no TSan reports. System JSC itself is not instrumented.
• A bounded ASan/UBSan subprocess against pristine upstream 839b352 reproduced post-destruction dispatch failure: mutex lock failed: Invalid argument. No ASan UAF diagnostic was emitted.
• git diff --check passed. No npm/polyfill suite or other engines were tested.

Provenance

Adapted from Matt Hargett's integration commits 7093c83d3a29020fc52f070859b7f7514ed3195f and 0a5d570d156d75f5fc01162f5fe6a7ccc891e3c3. This standalone commit preserves the original author/date and adds explicit host shutdown ordering, unlocked capture release, callable compatibility, and bounded tests on upstream main.

Note: this latent issue was discovered by the ChatGPT DayBreak model when I was doing adversarial testing on my webgpu branch.

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[matthargett]

Also validated this change in the integrated browser-polyfill stack on M4 Max/system JavaScriptCore with JSC_collectContinuously=1: all 55 native tests passed, including 362 JavaScript cases and the worker WPT subset (three existing JavaScript skips). The standalone TSan results remain recorded above.