Cancel .task work when the view disappears

[colemancda]

.task now cooperatively cancels its async closure when the view leaves the tree, matching SwiftUI. Previously it launched a Task on appear that ran forever.

Mechanism

• The .task modifier registers two callbacks — start and cancel — and the launched Task is stored in a registry keyed by the view's stable identity path, not by the callback id.
• The interpreter fires start on appear (DisposableEffect enter) and cancel on onDispose.

Why key by identity path

Callback ids are generation-scoped and evicted after one cycle, but a view can re-evaluate many times between appear and disappear — so a captured cancel id would be long gone by dispose. Keying the running Task by identity path (stable across every re-evaluation) makes cancel find the right task regardless. The interpreter also hands onDispose the freshest cancel id via a remembered holder, so the callback itself stays resolvable.

The user's closure observes cancellation the normal way (Task.sleep/Task.isCancelled throw/return on cancel).

Verification

• swift test — new start/cancel test (launches then cancels via the two ids, asserts the registry clears), 67 total passing
• Emulator (Interaction screen, new ".task (cancels on disappear)" example, ticks a counter every 0.5s while visible):
  • Show → counter climbs
  • Hide → counter frozen at 32 across a controlled 3.5s window (zero new ticks — the background loop really stopped)
  • Show again → counter resumes (36 → 45), so start-after-cancel works too