feat: Client auto-sense fails if a DTLS 1.3 `ServerHello` is fragmented (e.g., PQ keys + low MTU)

[xnorpx]

Description:
The client-side DTLS version auto-sense relies on a lightweight, stateless packet sniffer (server_hello_version_inner in src/detect.rs) to inspect the server's first response. This parser assumes the supported_versions extension is fully present in the first arrived UDP datagram.

It does not process DTLS fragmentation. Specifically:

1. It does not check fragment_offset.
2. If ext_end > body.len() (truncation due to fragmentation), it instantly aborts the search and falls back to DTLS 1.2.

Because the subsequent fragments belong to a 1.3 handshake, the resulting Client12 state machine throws a parse error and drops the connection.

Likelihood & Impact:

• Standard Cryptography: Likelihood is near 0%. Standard ServerHello and HelloRetryRequest messages are tiny (~80-120 bytes) and will never fragment.
• Post-Quantum Cryptography: Likelihood is moderate on constrained networks. An ML-KEM-768 key share puts the ServerHello at roughly ~1168 bytes. While this fits standard IPv6 minimum MTUs (1280), if a client is on a network with a heavily constrained MTU (e.g., under 1150 bytes due to VPNs or IPsec tunnels), the router/server will fragment the ServerHello.

In these specific low-MTU + PQ scenarios, client auto-detect will consistently fail the handshake.

Why it's a gap (and hard to fix):
Unlike the server-side fix, we cannot easily delegate this to the Client13 engine because the client hasn't decided which version state machine to initialize. Fixing this would require building a stateful DTLS fragment reassembly buffer directly inside ClientPending just to read the extensions block.

Potential Mitigations:

1. Accept the limitation: Let the handshake predictably fail on ultra-low MTU networks when PQ cryptography is involved.
2. Stateful Sniffer: Add a small reassembly buffer to ClientPending that waits for all fragments of message_seq == 0 before making the version decision.

[xnorpx]

Not high priority, but filing to keep in mind