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)

crates/quicproquo-core/src/hybrid_kem.rs

@@ -159,11 +159,14 @@ impl HybridKeypair {
     }
 
     /// Serialise private key for MLS key store: x25519_sk(32) || mlkem_dk(2400).
-    pub fn private_to_bytes(&self) -> Vec<u8> {
+    ///
+    /// The returned value is wrapped in [`Zeroizing`] so secret key material
+    /// is securely erased when dropped.
+    pub fn private_to_bytes(&self) -> Zeroizing<Vec<u8>> {
         let mut out = Vec::with_capacity(HYBRID_PRIVATE_KEY_LEN);
         out.extend_from_slice(self.x25519_sk.as_bytes());
         out.extend_from_slice(self.mlkem_dk.as_bytes().as_slice());
-        out
+        Zeroizing::new(out)
     }
 
     /// Reconstruct a hybrid keypair from private key bytes (from MLS key store).