fix(cable): close hybrid transport spec gaps in tunnel and L2CAP

[AlfioEmanueleFresta]

Problem

The hybrid (caBLE) transport in transport/cable handles the common QR-initiated flow, but four behaviours required by the CTAP hybrid / caBLE v2 spec are missing or broken, across tunnel connect handling and L2CAP framing.

Why it matters

These break interop with real authenticators and platforms. Redirecting tunnel servers are unreachable, dead linking records are retried forever, clean shutdown and late linking updates are lost, and some direct-BLE messages get corrupted.

What to do

1. Follow HTTP redirects on tunnel connect

tunnel::connect() (libwebauthn/src/transport/cable/tunnel.rs) calls tokio_tungstenite::connect_async(), which doesn't follow redirects, so any 3xx becomes ConnectionFailed. The spec requires following them.

• Follow 3xx redirects (e.g. 307/308) to the Location target before the WebSocket upgrade, with a redirect cap.
• Re-attach Sec-WebSocket-Protocol: fido.cable, and X-caBLE-Client-Payload for known-device connections, on the redirected request.

2. Handle HTTP 410 Gone on state-assisted contact

connect() collapses every non-101 status into ConnectionFailed. For CableTunnelConnectionType::KnownDevice (/cable/contact/{contact_id}), a 410 means the linking record is permanently gone, but nothing deletes it, so the dead link is retried forever.

• Surface 410 distinctly. No TransportError variant exists yet (transport/error.rs).
• On 410, delete the record via CableKnownDeviceInfoStore::delete_known_device and stop retrying. connect_data_channel() in connection_stages.rs needs store access (today it is only in the post-handshake connection() path).

3. Send Shutdown on close and linger for late linking data

CableChannel::close() (channel.rs) is an empty TODO and Drop just aborts. We handle inbound CableTunnelMessageType::Shutdown (protocol.rs, RecvOutcome::PeerShutdown) but never send one, and tearing down right after the CTAP response drops any post-transaction Update linking message.

• On close(), send a Shutdown frame over the encrypted channel before dropping it.
• After the CTAP response, linger briefly (~2 minutes) to capture late Update linking data.

4. Fix ambiguous CRLF framing over L2CAP

l2cap.rs ends each message with EOM = [0x0D, 0x0A] and split_next_message() splits at the first CRLF, but binary Noise/AES-GCM payloads can contain 0x0D 0x0A. For a 700-1400 byte ciphertext that is a ~1-2% chance of an internal CRLF corrupting the frame. The split_handles_binary_payload test only covers payloads with no internal CRLF.

• Make L2CAP message boundaries unambiguous for arbitrary binary payloads.
• Add a regression test for a payload containing an internal 0x0D 0x0A.
• Raise the framing ambiguity with the CTAP hybrid spec editors. Delimiter framing without escaping or a length prefix is unsafe and likely needs a spec clarification.