fix(runtime): prevent Cmd+W close crash during session despawn

[forketyfork]

Issue

Hotfix fast-path: no GitHub issue exists yet.
Issue linkage is intentionally omitted for now and will be added during mandatory cleanup.

Solution

This change fixes an intermittent crash when closing a terminal with Cmd+W.

The close path was using full session teardown while the event loop could still deliver a pending process-watcher callback. That left a small window where callback context could be freed too early.

To close that gap, runtime close now uses a dedicated despawn path that keeps active wait-context memory alive until callback cleanup can happen safely. Final deinit still does immediate teardown when the app is shutting down.

I also added a regression test that exercises this callback-safety behavior, and updated architecture docs to include the new session lifecycle API.

Test plan

• Launch Architect, open multiple terminals, press Cmd+W repeatedly on different sessions, and confirm the app does not crash.
• In full view, close the focused session with Cmd+W and confirm collapse/reflow behavior still works as before.
• Trigger the "Delete Terminal?" confirmation by running a foreground process, confirm delete, and verify no crash during/after close.

[Copilot]

Pull request overview

This PR fixes an intermittent crash when closing a terminal with Cmd+W by introducing a safe session teardown path for runtime closes. The crash occurred because session close used full teardown (deinit) while the event loop could still deliver pending process-watcher callbacks, creating a window where callback context could be freed prematurely.

Changes:

• Added despawn() method for runtime session closes that defers destruction of active wait contexts until callbacks can reclaim them
• Replaced deinit() with despawn() in Cmd+W close handlers
• Added regression test for callback safety during session despawn

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/session/state.zig	Introduced WaitContextCleanup enum, new despawn() method, refactored deinit() to use shared teardown() with conditional wait context cleanup, added regression test
src/app/runtime.zig	Updated Cmd+W close handlers and UI despawn action to use despawn() instead of deinit()
docs/ARCHITECTURE.md	Added despawn() to session lifecycle API documentation

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