fix(ai-sessions): give the catch-up failure path the same supersession answer

The catch block was the one probe-less way to arm a retry: a catch-up
whose loadPastChat threw after a newer turn-end queued would schedule a
retry carrying its stale chat id, which could then supersede the newer
catch-up and re-adopt — and re-persist — the chat the driver rotated
away from. The catch now makes the same supersession check the
'unavailable' path already sits behind, so a superseded failure leaves
the gate and the retry to the catch-up that owns them.

Also makes the latestWins failure case actually fail: both run calls
were synchronous, so the throwing task was skipped as superseded and the
catch went unexercised. A tick between them lets it throw, and the suite
now asserts the error was seen as well as the follow-up running.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019N8x51GqABUEFvg52V6aYo

frontend/src/lib/components/sessions/latestWins.test.ts

@@ -72,10 +72,14 @@ describe('createLatestWins', () => {
 			q.run('s', () => {
 				throw new Error('boom')
 			})
+			// Let the throwing task actually run before the next one queues, or it
+			// is skipped as superseded and the failure never happens.
+			await tick()
 			q.run('s', () => {
 				ran.push('after')
 			})
 			await tick()
+			expect(errorSpy).toHaveBeenCalled()
 			expect(ran).toEqual(['after'])
 		} finally {
 			errorSpy.mockRestore()

frontend/src/lib/components/sessions/sessionRuntime.svelte.ts

@@ -1189,8 +1189,11 @@ async function applyTurnEnd(
 		caughtUp = true
 	} catch (e) {
 		// A read that threw leaves the same mismatched pair an 'unavailable' one
-		// does, so it earns the same answer rather than a silent release.
+		// does, so it earns the same answer rather than a silent release — the
+		// supersession check included, or the retry would carry this task's stale
+		// chat id past the newer catch-up that owns the gate now.
 		console.error('sessionRuntime: catch-up failed', e)
+		if (superseded()) return
 		scheduleCatchUpRetry(sessionId, chatId, attempt)
 	} finally {
 		if (caughtUp) noteCaughtUp(sessionId)