v0.261.0 — the controller no longer swaps itself out from under an app update (R-608, R-609)

The controller self-updates daily at 04:30 by default, and after any hub report
once a floor sits above the box. That swap restarts the controller container.
The window proposed for automatic app updates is 02:30-05:00. It contains 04:30.

R-608 — a two-way lock, wired in main.go (stacks never imports selfupdate):
- stacks.Manager.AnyUpdating() -> Updater.SetAppUpdatingCheck, consulted in the
  same three places as the existing backupRunning gate.
- Updater.IsUpdateRunning -> Manager.SetSelfUpdatingCheck; UpdatePreflight
  refuses `self_updating`.
- MEASURED: the gap was narrower than assumed. The update's `backing-up` phase
  already takes the backup single-flight, so that one phase was covered. The
  other six were not, and `starting`/`verifying` are where data may have moved.
- The lock must NOT latch: a held app does not block the controller's own
  updates, including the release that might fix the hold.

R-609 — the 409 carries `data.reason`, additively. transient (busy, updating,
deploying, migrating, self_updating) vs terminal (held, downgrade). Found while
writing the test: the router refuses a HELD app on its own line before the
preflight, so `held` would have been the one reason missing.

Five red-proofs, each seen to fail.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/scripts/i18n_go_keys.json

@@ -1,6 +1,8 @@
 {
   "_comment": "Localisation slice 2 (R-557). key -> the base-commit Go literal it replaced, or the ORDERED list of literals a concatenation joined. Checked by scripts/i18n_go_parity.py against scripts/i18n_go_base.json, which is frozen at 736f54b49610 (the base commit of release v0.252.0). A key whose Hungarian text is not byte-identical to what the Go code said fails the gate.",
   "_preexisting": {
+    "err.selfupdate.alkalmazas_frissites_folyamatban": "BORN AS A KEY, v0.261.0 (R-608) -- the controller refuses to swap itself while a guarded app update is in flight. A NEW sentence, never a Go literal. Pinned by TestR608_TriggerUpdateRefusedWhileAnAppUpdates, which asserts the key itself.",
+    "err.stacks.update_self_updating": "BORN AS A KEY, v0.261.0 (R-608) -- the app update refuses to start while the controller is swapping itself. A NEW sentence, never a Go literal. Pinned by TestR608_PreflightRefusesWhileTheControllerSwaps, which asserts the refusal carries a Cause so errText can render it.",
     "badge.update.ahead.title": "BORN AS A KEY, v0.260.0 (R-524) -- a NEW sentence, never a Go literal, so there is nothing in the base capture to measure it against. Pinned in both languages by TestUpdateBadgeFollowsTheLanguage.",
     "err.stacks.update_downgrade": "BORN AS A KEY, v0.260.0 (R-524) -- the downgrade refusal. A NEW sentence, never a Go literal. Pinned by TestR524_PreflightRefusesDowngrade, which asserts the refusal carries a Cause so api.Router.errText can render it.",
     "func.state.running": "slice 0 -- pinned by TestLocaleFuncsHungarianBundleMatchesFuncMap",