Recover Live Activity after APNs 410 token expiry

[bjorkert]

Summary

A 6.1.0 user reported the Live Activity vanishing overnight and
refusing to come back without a manual Restart. Log trace:

• 04:12 — scheduled renewal succeeds, new LA adopted.
• 04:42 — APNs token expired (410) — restarting Live Activity,
  handleExpiredToken ends the activity.
• 04:42 – 08:42 — activities=1, current=nil, renewBy=0.0; every
  foreground entry logs no action needed (not in renewal window).
  No new LA is created.
• 08:42 — .dismissed: endingForRestart=false, renewalFailed=false, pastDeadline=false, renewBy=0.0 → classified as user swipe,
  dismissedByUser=true latches. From here every refresh logs
  LA update skipped — dismissedByUser=true until the user opens
  the app and taps Restart.

Root cause

Two cooperating bugs around an app-initiated end():

1. end() nulls current and clears laRenewBy. handleExpiredToken's
   comment said the LA would restart on the next BG refresh via
   refreshFromCurrentState(), but renewIfNeeded short-circuits when
   current is nil, and performRefresh's bind-existing path then
   rebinds to the just-ended activity (still listed in
   Activity<>.activities until iOS purges it). bind() clears
   endingForRestart, so the late .dismissed reads as renewBy=0 /
   renewalFailed=false / endingForRestart=false — branch (c) "USER"
   in the classifier.

2. The classifier had no way to recognize a stale observer firing for an
   activity the app no longer tracks.

Fixes

• handleExpiredToken drives the restart synchronously on iOS 17.2+
  (attemptPushToStartCreate(reason: "expired-token")), so the orphaned
  post-410 state is short-lived and adoption of the fresh activity
  cancels the old observer via attachStateObserver.
• performRefresh / update bind-existing only to .active
  activities. Binding to an .ended/.dismissed corpse would clear
  endingForRestart and re-attach an observer that only ever delivers
  .dismissed.
• .dismissed classifier gains branch (d): stale observer. If the
  dismissed activity is not the one we currently track, log and take no
  action. Only the foreground LA can be user-swiped, so a stale-observer
  delivery for an already-replaced activity must not latch
  dismissedByUser=true. The wasCurrentActivity signal is captured
  before current is cleared so legitimate user swipes still classify
  correctly.