release(v1.7.6-alpha): robust apply_update + manifest-override env var

apply_update frontend swap
  Transient EROFS on .198 (filesystem hiccup — root FS mounts with
  errors=remount-ro so a fleeting glitch can bounce /opt to RO for a
  moment) caught the pre-cleanup `rm -rf web-ui.new web-ui.bak` mid-
  stride and aborted the apply. Rewrote the swap to use a timestamped
  staging dir (web-ui.new.<ms>) and a timestamped old-copy path so
  nothing needs to be rm'd before the extract. After the new tree is
  mv'd into place, the previous rollback copy is rotated aside with a
  .<ms> suffix (best-effort) and this apply's old copy becomes the new
  web-ui.bak. If the final mv fails, the staged old is restored so
  nginx keeps serving.

handle_update_check manifest override
  handle_update_check takes the git path whenever ~/archy/.git exists.
  On the dev box (.116) that meant the Pull & Rebuild button was
  always the only option even though the manifest-path OTA was
  already wired via ARCHIPELAGO_UPDATE_URL. Now: if that env var is
  set, we skip the git detection entirely and use the manifest path.
  The regular fleet (no env var, no repo) hits the manifest branch
  naturally; beta dev nodes (repo + no env var) still get Pull &
  Rebuild; dev nodes with the env var explicitly set can finally test
  the manifest OTA end-to-end.

Artefacts:
  archipelago                                      356e78cc…91a6dd  40372288
  archipelago-frontend-1.7.6-alpha.tar.gz          4fb79664…0172e9  76984615 (reused)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/Cargo.lock

@@ -80,7 +80,7 @@ checksum = "a23eb6b1614318a8071c9b2521f36b424b2c83db5eb3a0fead4a6c0809af6e61"
 
 [[package]]
 name = "archipelago"
-version = "1.7.5-alpha"
+version = "1.7.6-alpha"
 dependencies = [
  "anyhow",
  "archipelago-container",