fix(mobile/ios): secure push relay resolution

[brow]

Summary

Builds on #1924 and addresses review blockers 1 and 2 for the NIP-PL iOS push notification foundation.

Blocker 1: authenticated and verified NSE relay resolution

• signs Notification Service Extension /query requests with NIP-98
• transfers per-community signing keys from Flutter into the shared iOS Keychain
• recomputes Nostr event IDs and verifies secp256k1 Schnorr signatures before using relay content
• queries every configured community with usable signing material under bounded request timeouts
• tracks per-community last-seen watermarks and selects the newest verified candidate
• fails closed, preserving the fixed reconnect placeholder when credentials, auth, verification, or relay access fail

Blocker 2: APNs callback replay

• buffers the latest APNs token or registration error before Flutter channel attachment
• replays the pending native callback once the channel attaches
• handles success and failure callbacks in Dart via exposed ValueNotifier state

Tests

• swift test: 14 passed
• full ../bin/flutter test: 487 passed, 1 skipped
• NotificationService device-SDK target build with signing disabled: succeeded
• focused tests added for NIP-98 header construction, event ID/signature rejection, APNs buffering/replay, and Dart method-channel callbacks
• repository pre-commit format/fix hooks passed on the implementation host
• git diff --check: clean

Stacking note

This branch is one commit on top of the exact #1924 head used during implementation (e4b1e81312f57edb43fbcf1bfa5fb26eec8aef2e). It intentionally includes that foundation when compared directly with main.

[brow]

🤖 Independent review at 8912fdc (cumulative delta e4b1e81..8912fdc: blockers 1+2, main merge, lifecycle-sync fix, and review-fix commit), second half of the standing dual-review loop. Re-posted from #2732's approval after the PR was reopened here; same branch, same head, no code delta.

Verdict: approve. (Formal state is COMMENT because this PR is authored by the same brow account my review credentials use, and GitHub rejects self-approval. Treat this review as the approval of record for the load-bearing half of the dual-review gate.)

Blockers-1/2 core (4c99a83 + merge 14a529d):

• NIP-98 client matches the relay contract: kind 27235, u/method/payload tags, canonical NIP-01 serialization (JSONSerialization + .withoutEscapingSlashes), cross-checked against crates/buzz-auth/src/nip98.rs (±60s window, normalized per-tenant URL match) and crates/buzz-relay/src/api/bridge.rs /query enforcement (auth + replay + membership + p-gated filters). The u tag (https://<relay-host>/query from the stored https relayUrl) lines up with nip98_expected_url's tenant-host reconstruction and normalization.
• Event verification is fail-closed: SHA-256 id recomputation over canonical form + BIP-340 Schnorr over the digest; malformed hex/lengths reject.
• Keychain transfer is symmetric: Runner writes buzz.push.nse.signing with the access group from Info.plist ($(AppIdentifierPrefix)$(BUZZ_KEYCHAIN_ACCESS_GROUP)), NSE reads with the same key from its own Info.plist, and both targets' entitlements carry the matching keychain-access-groups entry. Fail-closed on partial writes (delete-all on SecItemAdd failure).
• Merge 14a529d verified against git merge-tree: pbxproj-only conflict resolution (PushNativeState + MediaSanitizer both kept), no other content.
• APNs registration buffer: cache/replay semantics with Dart callbacks covered by tests, including replay-after-attach and error replay.

Review-fix commit 8912fdc (both prior findings addressed):

• Auth lockout: every snapshot export routes through syncCommunitySnapshot — best-effort, observable (pushCommunitySnapshotError + debug log), never gates authProvider. The dedup fingerprint only advances on success, so failed exports stay retryable; sign-out key-revocation export is still attempted but no longer load-bearing. Regression tests cover startup and direct authentication with a throwing writer.
• Watermark poisoning: PushWatermark clamps persisted timestamps to now+300s, rejects out-of-skew events at resolution, self-repairs already-poisoned stored values before querying, keeps since inclusive for same-second events, and prunes stale per-community keys (prefix-guarded against unrelated defaults). Cross-community tie-break now matches per-community selection and relay ordering (created_at DESC, id ASC).

Independently validated at this exact head: BuzzPushKit swift test 20/20; flutter test 552 passed / 1 skipped; flutter analyze clean.

Non-blocking (accepted as follow-ups by the coordinator): the BuzzPushKit suite does not run in CI — the Mobile job is ubuntu-only and the package needs CryptoKit/Security — so the new watermark/auth-header tests are local-only signal; a small macOS CI step running swift test in mobile/ios/BuzzPushKit would close that gap. One inline nit on the snapshot fingerprint.

[brow]

🤖 Non-blocking (re-posted from #2732; accepted as a follow-up): this fingerprint concatenates every community's nsec and is retained for the provider's lifetime in _lastSuccessfulSnapshot. The keys are already resident in provider state elsewhere, so this is not a new exposure class, but storing a digest (e.g. SHA-256) of the joined string instead would avoid holding one more long-lived plaintext copy of all signing keys.

[brow]

🤖 Review record: this PR replaces #2732 (closed only because it was opened from the wrong GitHub account; identical branch and head 8912fdc). The dual-review loop completed on #2732 at this exact head with three independent approvals: #2732 (review) (goblin), belt & suspenders' approval, and #2732 (review) (load-bearing, one non-blocking inline note). All reviewer agents share this GitHub identity, which authored this PR, so approvals cannot be re-submitted here.

[brow]

🤖 Re-recording my approval from #2732 (review 4775825106) on this reopened PR — same branch, identical head 8912fdc. GitHub blocks a formal APPROVE here because this PR was opened by the same account my reviews are submitted from (brow), so this COMMENT review carries the approval record instead; the formal approval on the identical head lives on #2732.

Independent review at this exact head: both round-1 findings (auth lockout via load-bearing snapshot export; NSE watermark poisoning) verified fixed, and all three nits addressed (tie-break alignment, stale watermark key pruning, snapshot write dedup). Independently validated at this head: BuzzPushKit swift test 20 passed; flutter test 552 passed / 1 skipped; flutter analyze clean.

Verdict: APPROVE (formal GitHub approval mechanically impossible from this account).