fix: security hardening — 40 findings from full codebase review

Full codebase review by 4 independent agents (security, architecture,
code quality, correctness) identified ~80 findings. This commit fixes 40
of them across all workspace crates.

Critical fixes:
- Federation service: validate origin against mTLS cert CN/SAN (C1)
- WS bridge: add DM channel auth, size limits, rate limiting (C2)
- hpke_seal: panic on error instead of silent empty ciphertext (C3)
- hpke_setup_sender_and_export: error on parse fail, no PQ downgrade (C7)

Security fixes:
- Zeroize: seed_bytes() returns Zeroizing<[u8;32]>, private_to_bytes()
  returns Zeroizing<Vec<u8>>, ClientAuth.access_token, SessionState.password,
  conversation hex_key all wrapped in Zeroizing
- Keystore: 0o600 file permissions on Unix
- MeshIdentity: 0o600 file permissions on Unix
- Timing floors: resolveIdentity + WS bridge resolve_user get 5ms floor
- Mobile: TLS verification gated behind insecure-dev feature flag
- Proto: from_bytes default limit tightened from 64 MiB to 8 MiB

Correctness fixes:
- fetch_wait: register waiter before fetch to close TOCTOU window
- MeshEnvelope: exclude hop_count from signature (forwarding no longer
  invalidates sender signature)
- BroadcastChannel: encrypt returns Result instead of panicking
- transcript: rename verify_transcript_chain → validate_transcript_structure
- group.rs: extract shared process_incoming() for receive_message variants
- auth_ops: remove spurious RegistrationRequest deserialization
- MeshStore.seen: bounded to 100K with FIFO eviction

Quality fixes:
- FFI error classification: typed downcast instead of string matching
- Plugin HookVTable: SAFETY documentation for unsafe Send+Sync
- clippy::unwrap_used: warn → deny workspace-wide
- Various .unwrap_or("") → proper error returns

Review report: docs/REVIEW-2026-03-04.md
152 tests passing (72 core + 35 server + 14 E2E + 1 doctest + 30 P2P)

playbooks/group-lifecycle.yaml

@@ -0,0 +1,39 @@
+name: "Group Lifecycle"
+description: "Create a group, invite a member, send messages, inspect, leave"
+variables:
+  group_name: "test-group"
+  member: "bob"
+
+steps:
+  - command: create-group
+    args:
+      name: "$group_name"
+
+  - command: invite
+    args:
+      target: "$member"
+    on_error: continue
+
+  - command: members
+
+  - command: group-info
+
+  - command: send
+    args:
+      text: "First group message"
+
+  - command: send
+    args:
+      text: "Second group message"
+
+  - command: history
+    args:
+      count: 10
+
+  - command: assert
+    condition: message_count
+    op: gte
+    value: 2
+
+  - command: leave
+    on_error: continue