noise: implement inlined stream-muxer negotiation via NoiseExtensions

[adust09]

Spec

specs/connections/inlined-muxer-negotiation.md, Multiplexer Negotiation over Noise (1A Working Draft).

Noise handshake messages 2 and 3 may carry NoiseExtensions listing supported stream multiplexers. The selected muxer is the first initiator preference supported by both peers; if the optimization is used and there is no overlap, the handshake must fail.

Current state

The Noise payload decoder intentionally skips protobuf field 4 (extensions) as an unknown field. The upgrade pipeline always performs a separate multistream-select negotiation for /yamux/1.0.0 after Noise, costing an additional round trip.

This is a backwards-compatible optimization rather than a defect in the existing Noise + Yamux path.

Scope

• Add the NoiseExtensions protobuf representation and codec.
• Send supported muxers in the appropriate XX handshake payloads.
• Select the muxer using initiator preference order.
• Surface the selected muxer from the Noise handshake to Switch.Upgrade.
• Skip post-handshake muxer multistream-select only when inlined negotiation succeeds.
• Preserve fallback interoperability with peers that omit extensions.
• Fail the optimized negotiation when both sides send extensions but have no common muxer, as specified.

Acceptance tests

• Byte-level codec vectors for NoiseExtensions.
• Initiator preference determines the selected common muxer.
• Successful inline selection avoids the extra multistream-select exchange.
• A peer without extensions follows the existing post-handshake negotiation path.
• No-overlap behavior matches the working-draft requirement.

Priority

Low: the specification is a Working Draft and the existing path is interoperable; this primarily removes one connection-setup RTT.