feat(voice): DAVE E2E + full voice UX (squash of voice/dave-recv)

Squashed branch: voice/dave-recv → master. Closes Pas 12 (DAVE E2E) and lands
voice-mode UX polish + verbal voice control on top of the Pas 1-10 scaffolding
already on master.

## DAVE E2E receive-side decrypt (e4f3177)

Vendored fork: discord-ext-voice-recv 0.5.3a+echo.dave1. Patches the receive
pipeline to handle Discord's mandatory DAVE encryption on voice gateway v=8.
- `_maybe_dave_decrypt`: uses davey.can_passthrough(user_id) as primary gate,
  falls through to dave.decrypt for DAVE-epoch peers, drops on decrypt failure
  without killing the reader thread.
- VAD fix: silero-vad v5+ requires exactly 512 samples; our 100ms window
  (1600 samples) was silently raising ValueError → STT never fired. Now slice
  into 512-sample chunks.
- Whisper: bumped beam_size 1→5 and added RO initial_prompt.
- Tests: 11 DAVE unit tests + 2 callback integration tests + contract test
  with fork-version guard.

## Voice UX polish (d1bc77e)

- Killed the 3s "mă gândesc" filler (always collided with Claude p50 4-7s).
- Barge-in via `ttsq.clear()` at top of `on_segment_done`.
- DTX silence-flush poller (200ms tick) — Discord stops sending RTP packets
  when silent, so the inline silence-check in sink.write() never fired for
  trailing audio; background thread handles it.
- `EchoStreamingAudioSource.read()` non-blocking — old `get_frame(timeout=0.1)`
  wrecked Discord's 20ms cadence and the client interpreted bursts as
  stuttering (Marius heard "4 de minute" instead of full sentence).
- RO time expansion: 23:09 → "douăzeci și trei și nouă minute".
- Supertonic Unicode sanitize centralized in tools/tts.py.
- Whisper local_files_only=True — no HF metadata GET on each startup.
- Diagnostic logging through sink → VAD → Claude stream → TTS chain.

## Voice mode iteration (e589e48)

- `personality/VOICE_MODE.md` — voice-tailored system prompt (short, no
  markdown, no abbreviations, time without seconds, distances in
  "mii"/"milioane"); plumbed via build_system_prompt(voice_mode=True).
- Isolated voice session key `voice:<channel_id>` — voice doesn't share
  context with text adapter on the same channel; auto-applied without
  /clear ceremony. /clear drops both keys.
- Metric units + Romanian thousands (normalize.py): "384.000 km" →
  "trei sute optzeci și patru de mii de kilometri" with feminine-correct
  pluralization and "de" particle for ≥20.
- `/voice setvoice <M1-F5>` slash command with native autocomplete; swaps
  live + persists voice.default_voice to config.json.
- Verbal voice change (src/voice/voice_commands.py + 29 tests) — "schimbă
  vocea pe M5", "voce em cinci", with permissive substring fallback for
  Whisper-mangled forms like "Mâcinci"=M5 and "unul cinci"=M5. Whisper
  initial_prompt now lists voice vocabulary to bias STT toward clean
  outputs.
- Fast barge-in: VAD ≥2 consecutive windows (~200ms) on Marius's user
  while Echo has pending TTS frames → cut him off mid-sentence so user
  doesn't wait the full silence + STT cycle. Acoustic echo bleed-through
  still requires headphones (no AEC).

## Test suite

130 voice + router tests pass (test_voice_recv_dave, test_voice_session_cleanup,
test_voice_adapter_contract, test_voice_normalize, test_voice_commands,
test_router).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/router.py

@@ -123,8 +123,10 @@ def route_message(
     # Text-based commands (not slash commands — these work in any adapter)
     if text.lower() == "/clear":
         default_model = _get_config().get("bot.default_model", "sonnet")
-        cleared = clear_session(channel_id)
-        if cleared:
+        cleared_text = clear_session(channel_id)
+        # Also drop the isolated voice session if one exists on this channel.
+        clear_session(f"voice:{channel_id}")
+        if cleared_text:
             return f"Session cleared. Model reset to {default_model}.", True
         return "No active session.", True
 
@@ -159,12 +161,19 @@ def route_message(
     # (Engineering decision #14 in the plan.) Only the discord-voice adapter
     # triggers it — text adapters keep the message verbatim.
     claude_text = text
-    if adapter_name == "discord-voice":
+    voice_mode = adapter_name == "discord-voice"
+    if voice_mode:
         user_name = _get_config().get("voice.user_name", "user") or "user"
         claude_text = f"[speaker:{user_name}] {text}"
+    # Voice sessions use an isolated session key so they start fresh with
+    # VOICE_MODE.md and don't pollute the text channel's conversation.
+    session_key = f"voice:{channel_id}" if voice_mode else channel_id
 
     try:
-        response = send_message(channel_id, claude_text, model=model, on_text=on_text)
+        response = send_message(
+            session_key, claude_text, model=model, on_text=on_text,
+            voice_mode=voice_mode,
+        )
         _set_last_response(channel_id, response)
         return response, False
     except Exception as e: