Fix CanvasLayoutStore teardown crash on older Swift runtimes

[onevtail]

Summary

Add an explicit empty nonisolated deinit to CanvasLayoutStore to avoid the TaskLocal invalid-free crash in older system Swift concurrency runtimes (Sentry PROWL-MACOS-KB / PROWL-MACOS-KC).

Root cause

Under the default main-actor isolation, the compiler infers an isolated deinit for CanvasLayoutStore. Its destructor calls swift_task_deinitOnExecutor. On older system runtimes (for example macOS 26.3.1), a synchronous release while TaskLocal storage is bound hits the invalid free fixed upstream in swiftlang/swift#85204. The fix is in the runtime, so a newer Xcode does not help: the app links the system libswift_Concurrency.dylib.

The published 2026.9.25 build (Xcode 27, 27A266a) still branches to swift_task_deinitOnExecutor from the CanvasLayoutStore heap destructor.

The store has no actor-bound teardown work. Persistence happens on mutation, not on destruction. State access stays main-actor isolated.

Changes

• CanvasLayoutStore: explicit empty nonisolated deinit.
• CanvasLayoutStoreTests: release the store synchronously on a main-queue callback (no current Task) with a live TaskLocal binding. Check lifetime, binding restoration, and persistence.

Validation

• Extracted store on iOS 26.2 simulator (affected runtime): original source aborts with the same swift_task_deinitOnExecutorImpl / invalid-free stack; patched source completes 10,000 releases outside a Task.
• iOS 26.4 simulator: original and patched both complete 10,000 releases.
• SIL (Xcode 26.3 and 26.6): original destructor calls swift_task_deinitOnExecutor; patched destructor does not.
• make build-app, make check, and all 5 CanvasLayoutStoreTests pass (Xcode 26.6).

Not yet verified

• Build and tests with the release Xcode 27 toolchain and original dependency pins (local validation used Xcode 26.6; the lockfile rewrite was reverted and is not part of this PR).
• Sidebar switch / archive regression on macOS 26.3.1.

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