Context-compaction running toast is not settled on error / abort / replacement reconnect #4854

Follow-up split out of #3651, per @Astro-Han's review.

Problem

The running "Compacting context…" toast (apps/desktop/src/renderer/app-shell-context-compaction.ts, opened with duration: 0) is only dismissed when a complete event carries a contextCompactionOutcome. The other supported terminal / recovery paths do not carry that field, so the toast is left permanently visible after the compaction row has ended:

• user stops the compaction → abort
• a thrown backend failure → error
• a reconnect after a later root replaced this run → seedStoredTerminal

Why this is a follow-up rather than part of #3651

The fix has to settle the toast from the legacy AppShell: the presentation lives in app-shell-context-compaction.ts, and its error / abort wiring would go in the session-event dispatch (app-shell-session-events.ts + app-shell.tsx). The check-renderer-architecture --base monotonic-debt ratchet forbids any nonTriviaTokens increase on those legacyAppShell.files entries, and the dispatch has no non-legacy seam — so the wiring cannot land in #3651 without growing that debt.

Proposed approach

Extract the compaction presentation and its event handling out of the legacy AppShell into a feature module, then settle the specific running compaction on error and abort, and preserve the canonical outcome on replacement recovery (seedStoredTerminal). Regression cases to add: started → error, started → abort, and an old compaction replaced before reconnect.