fix(M2): wire vc_client's real voice plane through the C ABI, not just raw sockets

test_m2_voice passed against raw BSD sockets, but vc_client::stream_start/stop,
UDP binding, and capture/recv were still VC_ERR_NOT_IMPLEMENTED stubs -- meaning
vccli and any GUI client still couldn't actually talk. Implements the real
client-side UDP-binding handshake, media key derivation, capture->encode->seal->
send and recv->open->decode->playback paths, plus server-side StreamInfo
broadcast so peers learn about each other's streams via sync_remote_streams().

Adds test_voice_client_abi (two real vc_client instances, not raw sockets) and
vccli --voice/--mute/--text flags, manually verified live between two instances.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -4,10 +4,11 @@ Auto-loaded each session. This is the **map**: build commands, architecture at a
 where everything is. For the *working method* read [`AGENTS.md`](AGENTS.md); for *what's done
 and what's next* read [`PROGRESS.md`](PROGRESS.md); for *design* read [`docs/`](docs/).
 
-> **One-line status:** M2 voice/media plane is complete and verified (`ctest --preset m1-dev`
-> green — 9/9 tests including UDP relay, AEAD, Opus round-trip, and the M2 exit criterion
-> `test_m2_voice`). Next up is **M3** (multi-stream, per-channel tuning, listener-side NR).
-> See [`PROGRESS.md`](PROGRESS.md).
+> **One-line status:** M2 voice/media plane is complete and verified through the real client
+> library, not just a raw-socket harness (`ctest --test-dir build/m1-dev` green — 10/10 tests,
+> including `test_voice_client_abi` driving two real `vc_client`s end-to-end, and `vccli
+> --voice` manually verified live). Next up is **M3** (multi-stream, per-channel tuning,
+> listener-side NR). See [`PROGRESS.md`](PROGRESS.md).
 
 VoiceCat = self-hosted native voice & text chat (TeamSpeak/Mumble-style). Plain TCP (control)
 + UDP (media), no WebRTC, encrypted by default. A shared C++ core (`libvoicecat`) drives