Summary

The QUIC connection layer previously generated a random peer ID when the multiaddr lacked a p2p component (in `connect()`) and unconditionally on the server side (in `listen()` / `handle_handshake`). The comment at the synthesis sites explicitly flagged this as NOT correct for production. This PR replaces both fallbacks with real verification of the peer's libp2p TLS extension.

What changed

`tls.py` (+196 lines)

Adds the inverse of the existing certificate generator:

• `PeerVerificationError` — typed exception for libp2p TLS validation failures.
• `verify_libp2p_certificate(cert) -> PeerId` — finds the extension by OID `1.3.6.1.4.1.53594.1.1`, parses the ASN.1 `SignedKey ::= SEQUENCE { OCTET STRING, OCTET STRING }` envelope, decodes the protobuf `PublicKey { Type, Data }`, rejects unsupported key types, reconstructs the secp256k1 identity key, verifies the signature over `SIGNATURE_PREFIX || SubjectPublicKeyInfo DER`, and derives the canonical PeerId.
• Private ASN.1 / protobuf parsing helpers that accept the same three DER length forms used by the encoder.

`connection.py` (+61 / −18)

• `LibP2PQuicProtocol.init` — server-side instances set `self._quic.tls._request_client_certificate = True`. This is the same mechanism py-libp2p uses on top of aioquic; aioquic does not expose mTLS as a public configuration option.
• `connect()` — pulls the server cert from the completed TLS session, verifies it, and raises `QuicTransportError("Peer identity mismatch")` if the multiaddr's p2p component disagrees with the verified identity (MITM defense).
• `listen()` / `handle_handshake` — verifies the client cert; tears down the QUIC session if the cert is missing or fails verification, instead of registering a connection with a synthesized peer ID.

Test plan

• `uv run --group lint ruff check` on networking — clean
• `uv run --group lint ruff format --check` — clean
• `uv run --group lint ty check src/.../networking/ tests/.../networking/` — clean
• `uv run pytest tests/lean_spec/subspecs/networking/` — 814 passed

Test additions:

• 7 verifier tests in `test_tls.py`: round-trip, distinct keys, missing extension, tampered signature, malformed outer SEQUENCE tag, unknown KeyType, truncated envelope.
• `test_connection.py`: replaces the two old random-fallback `connect()` tests with four real-cert tests, including peer-ID mismatch and missing cert. Adds a matching pair for `listen()`.

Things worth a reviewer's eye

1. `_request_client_certificate` is a private attribute on aioquic's `tls.Context`. It is the only mechanism for mTLS in aioquic today and is what py-libp2p uses. If aioquic ever renames or drops it, the server will silently lose mTLS — worth a follow-up to pin aioquic and add a startup assertion.
2. No live socket test. The existing test suite never exercised a real network handshake; this PR keeps that style. A real end-to-end test (two `QuicConnectionManager` instances handshaking over 127.0.0.1) would be valuable as a follow-up.
3. Cross-client interop. `test_roundtrip_recovers_peer_id` only verifies our encoder/decoder agree. A future PR should vendor a known-good cert byte string from rust-libp2p as a test vector to catch divergence from the libp2p TLS spec.

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