release(v1.7.18-alpha): transitive peers default Trusted + update-flow logs

Flip transitively-discovered federation peers to Trusted instead of
Observer. Hints are already only ingested from peers we trust and only
peers we trust are re-exported via build_local_state, so the chain of
trust is already vetted end-to-end — making the user promote each
newcomer by hand was friction with no security win.

Backend:
- federation/sync.rs: merge_transitive_peers now inserts TrustLevel::Trusted
  (doc comment updated to explain the transitive-trust rationale)
- update.rs: info! log at download start (version, components, total_bytes,
  staging path), cancel (staging wiped?, marker cleared?), and apply (backup
  path) so journalctl reveals where a stuck update actually is

Frontend:
- SystemUpdate What's New block gets a v1.7.18-alpha entry

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 = "1.7.17-alpha"
+version = "1.7.18-alpha"
 dependencies = [
  "anyhow",
  "archipelago-container",