fix(frontend): release listeners and registries that outlive owners

[Harry19081]

Stacked on #830 (perf/heavy-component-leaks) → #829. Merge in order; this PR's diff is only the top commit.

Summary

Third frontend RAM pass: two thorough lifecycle sweeps (effects without cleanup, module-scope maps, registries holding DOM/xterm/EditorView, xterm/webview teardown, per-session atom families) over WorkStation, engines, hooks and scaffold. The codebase turned out disciplined; this PR fixes the genuine defects found — every change is behavior-neutral (nothing a user can see changes; only what is retained after it should have been released). Each has a regression test.

Problem

• SnapshotCacheManager.subscribeSession returned a disposer that captured the listener Set it was created with and deleted the registry entry whenever that Set emptied. evictSessionCache (reached by reload / manual compact / edit-message / sidebar delete while consumers are still mounted) drops the entry; a later subscriber installs a fresh Set, and the stale disposer then silently unregistered the live Set — mounted consumers stopped receiving es:changed pushes and pinned their last full snapshot.
• useSearchResults.loadMore unlistened its two Tauri listeners only on the success path; a rejected searchCodeStreaming (invalid regex, repo unmounted) left both — each closing over the entire current result set — registered for the process lifetime and running on every later search-result event.
• OSC-633 command-detection state (up to 200 command entries per terminal) was never pruned: removeCommandDetectionAtom had zero callers.
• Deleting a session disposed its tab workspace but not its session:<id> editor cache / active-repo pointer (heap + a localStorage blob parsed at boot).
• Chat/Communication/config.ts kept a module-scope single-slot memo of the last-built session's full SessionEvent[] and MessageEntry trees, outliving unmount / session switch / session close.
• A WebGL addon that threw inside loadAddon/activate was never disposed while its budget slot was returned (orphaned GL context; the 8-slot budget could be exceeded).
• TurnMetadataFooterSlot touched turnMetadataAtomFamily with sessionId ?? "", creating empty-session-id entries that the loader's GC never retains or removes.

Solution

• Disposer re-looks-up the current Set and only removes the registry entry when it is its own (snapshotCacheManager.ts).
• Listeners hoisted and released in finally (useSearchResults.ts).
• removeCommandDetectionAtom called from both terminal-removal paths (terminal/index.ts).
• New disposeEditorCacheForSessionAtom (editorCache.ts), wired into the single dispose callback both session-delete paths use (useWorkstationSidebarHandlers.ts).
• Memo → identity-keyed WeakMap (same hit semantics, no retention).
• webglAddon?.dispose() on the throw path in terminalSetup.ts and XtermOutput/index.tsx.
• TurnMetadataFooterSlot split into a wrapper (null/visibility gate) + body that only reads the family with a real session id.

Potential risks

• Low. The disposer change only affects the stale-disposer-after-evict case; the normal last-subscriber-removes-entry path is unchanged and tested. WeakMap memo hits are a superset of the old single-slot memo (never stale — keyed by array identity). Everything else is cleanup on close/error paths.
• Not changed (documented in the audit fix log as follow-ups): LRU-capping saved tab layouts across live sessions (would drop old sessions' tab state), cursorIdeTurnSummariesAtomFamily retention for browsed Cursor sessions (needs mount-gated GC), two ~100 B/session maps with reconcile semantics, and the unreferenced search/fileTrackingAtom.ts + search/cacheAtom.ts modules (dead-code deletion candidates).

Validation / Test plan

• pnpm typecheck clean; lint-staged (eslint + tsc) passed on commit.
• New tests: subscribeSessionDisposer.test.ts (stale disposer must not unregister a newer live Set), useSearchResults.loadMoreListeners.test.ts (both listeners released on rejection), commandDetectionLifecycle.test.ts (both removal paths prune), editorCache.test.ts dispose case (memory + storage), utils.test.ts memo identity/independence cases.
• 137 test files / 1 172 tests green across the touched areas (SessionCore store, terminal, XtermOutput, ChatHistory, Communication, EditorPrimarySidebar, NavigationSidebar, workstation store).
• Manual: reload / manual compact a session with subagents and confirm the chat keeps updating; trigger a failing search load-more then run further searches; open/close terminals; delete a session that had editor tabs.