feat(voice): Pas 4 — _discord_voice_adapter.py thin layer + contract test

Adapter layer peste vendored discord-ext-voice-recv. Re-exports:
VoiceReceiveClient, AudioSink, VoiceData, plus async helper
connect_voice(channel). Discord voice protocol e fragil, upstream e
hobby fork — dacă pică, swap la py-cord = doar acest fișier rescris.

Contract test (22 assertions) prinde drift la upgrade vendor:
- VoiceReceiveClient methods: connect/disconnect/listen/stop_listening/
  is_listening/stop/cleanup
- listen(sink, *, after=None) signature
- sink property (read/write)
- AudioSink methods: write/cleanup/wants_opus + write(self, user, data) arity
- VoiceData slots (packet/source/pcm) + .opus property

Critical pentru Lane PIPE downstream: write() e called from audio thread
(NOT asyncio loop) — threading primitives mandatory pentru EchoVoiceSink.

22/22 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/voice/_discord_voice_adapter.py

@@ -0,0 +1,67 @@
+# -*- coding: utf-8 -*-
+"""Adapter layer over `discord-ext-voice-recv` (vendored at vendor/).
+
+If discord-ext-voice-recv breaks, swap to py-cord by rewriting only this file.
+Contract test in tests/test_voice_adapter_contract.py guards drift.
+
+Downstream consumers (`src/voice/*`, `src/adapters/discord_voice.py`) MUST
+import from this file — never from `discord.ext.voice_recv` directly.
+
+## Public API surface (stable across upstream changes)
+
+- ``VoiceReceiveClient`` — alias for ``voice_recv.VoiceRecvClient``. Subclass
+  of ``discord.VoiceClient`` with extra audio-receive plumbing.
+  Key methods used by the pipeline:
+    * ``await client.disconnect(force: bool = False)`` (from discord.VoiceClient)
+    * ``client.listen(sink, *, after=None)`` — attach an ``AudioSink``;
+      raises ``discord.ClientException`` if not connected or already listening
+    * ``client.stop_listening()`` — detach the current sink
+    * ``client.is_listening() -> bool``
+    * ``client.stop()`` — stop both playing and listening
+    * ``client.sink`` (property, getter+setter) — swap the active sink in place
+
+- ``AudioSink`` — abstract base. Subclasses MUST implement:
+    * ``write(user: Optional[discord.User|Member], data: VoiceData) -> None``
+    * ``wants_opus() -> bool`` (True → receive opus bytes; False → receive PCM)
+    * ``cleanup() -> None``
+
+- ``VoiceData`` — per-packet container. Slots: ``packet``, ``source``, ``pcm``.
+  ``.pcm`` is decoded 48kHz s16le stereo bytes when ``wants_opus()`` is False.
+  ``.opus`` property returns the raw opus bytes from the underlying RTP packet.
+
+- ``connect_voice(channel) -> VoiceReceiveClient`` — async helper, returns a
+  connected receive-capable voice client.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from discord.ext import voice_recv
+
+if TYPE_CHECKING:
+    import discord
+
+
+# --- Stable re-exports -------------------------------------------------------
+
+VoiceReceiveClient = voice_recv.VoiceRecvClient
+AudioSink = voice_recv.AudioSink
+VoiceData = voice_recv.VoiceData
+
+
+__all__ = [
+    "VoiceReceiveClient",
+    "AudioSink",
+    "VoiceData",
+    "connect_voice",
+]
+
+
+async def connect_voice(channel: "discord.VoiceChannel") -> VoiceReceiveClient:
+    """Connect to a Discord voice channel with the receive-capable client.
+
+    Thin wrapper around ``channel.connect(cls=VoiceRecvClient)`` so callers
+    don't have to import the vendored class directly.
+    """
+    return await channel.connect(cls=VoiceReceiveClient)