feat: add Mumble music playback with Opus streaming

ctypes libopus encoder (src/derp/opus.py), voice varint/packet builder
and stream_audio method on MumbleBot (src/derp/mumble.py), music plugin
with play/stop/skip/queue/np/volume commands (plugins/music.py).
Audio pipeline: yt-dlp|ffmpeg subprocess -> PCM -> Opus -> UDPTunnel.
67 new tests (1561 total).

tests/test_mumble.py

@@ -11,6 +11,7 @@ from derp.mumble import (
     MSG_PING,
     MSG_SERVER_SYNC,
     MSG_TEXT_MESSAGE,
+    MSG_UDPTUNNEL,
     MSG_USER_REMOVE,
     MSG_USER_STATE,
     MumbleBot,
@@ -20,15 +21,19 @@ from derp.mumble import (
     _build_ping_payload,
     _build_text_message_payload,
     _build_version_payload,
+    _build_voice_packet,
     _decode_fields,
     _decode_varint,
     _encode_field,
     _encode_varint,
+    _encode_voice_varint,
     _escape_html,
     _field_int,
     _field_ints,
     _field_str,
     _pack_msg,
+    _scale_pcm,
+    _shell_quote,
     _strip_html,
     _unpack_header,
 )
@@ -912,3 +917,181 @@ class TestMumbleBotConfig:
         }
         bot = MumbleBot("test", config, PluginRegistry())
         assert bot._proxy is False
+
+
+# ---------------------------------------------------------------------------
+# TestVoiceVarint
+# ---------------------------------------------------------------------------
+
+
+class TestVoiceVarint:
+    def test_zero(self):
+        assert _encode_voice_varint(0) == b"\x00"
+
+    def test_7bit_max(self):
+        assert _encode_voice_varint(127) == b"\x7f"
+
+    def test_7bit_small(self):
+        assert _encode_voice_varint(1) == b"\x01"
+        assert _encode_voice_varint(42) == b"\x2a"
+
+    def test_14bit_min(self):
+        # 128 = 0x80 -> prefix 10, value 128
+        result = _encode_voice_varint(128)
+        assert len(result) == 2
+        assert result[0] & 0xC0 == 0x80  # top 2 bits = 10
+
+    def test_14bit_max(self):
+        result = _encode_voice_varint(0x3FFF)
+        assert len(result) == 2
+
+    def test_21bit(self):
+        result = _encode_voice_varint(0x4000)
+        assert len(result) == 3
+        assert result[0] & 0xE0 == 0xC0  # top 3 bits = 110
+
+    def test_28bit(self):
+        result = _encode_voice_varint(0x200000)
+        assert len(result) == 4
+        assert result[0] & 0xF0 == 0xE0  # top 4 bits = 1110
+
+    def test_64bit(self):
+        result = _encode_voice_varint(0x10000000)
+        assert len(result) == 9
+        assert result[0] == 0xF0
+
+    def test_negative_raises(self):
+        import pytest
+        with pytest.raises(ValueError, match="non-negative"):
+            _encode_voice_varint(-1)
+
+    def test_14bit_roundtrip(self):
+        """Value encoded and decoded back correctly (manual decode)."""
+        val = 300
+        data = _encode_voice_varint(val)
+        assert len(data) == 2
+        decoded = ((data[0] & 0x3F) << 8) | data[1]
+        assert decoded == val
+
+    def test_7bit_roundtrip(self):
+        for v in range(128):
+            data = _encode_voice_varint(v)
+            assert len(data) == 1
+            assert data[0] == v
+
+
+# ---------------------------------------------------------------------------
+# TestVoicePacket
+# ---------------------------------------------------------------------------
+
+
+class TestVoicePacket:
+    def test_header_byte(self):
+        pkt = _build_voice_packet(0, b"\xaa\xbb")
+        assert pkt[0] == 0x80  # type=4, target=0
+
+    def test_sequence_encoding(self):
+        pkt = _build_voice_packet(42, b"\x00")
+        # byte 0: header, byte 1: sequence=42 (7-bit)
+        assert pkt[1] == 42
+
+    def test_opus_data_present(self):
+        opus = b"\xde\xad\xbe\xef"
+        pkt = _build_voice_packet(0, opus)
+        assert pkt.endswith(opus)
+
+    def test_length_field(self):
+        opus = b"\x00" * 10
+        pkt = _build_voice_packet(0, opus)
+        # header(1) + seq(1, val=0) + length(1, val=10) + data(10) = 13
+        assert len(pkt) == 13
+        assert pkt[2] == 10  # length varint
+
+    def test_terminator_flag(self):
+        opus = b"\x00" * 5
+        pkt = _build_voice_packet(0, opus, last=True)
+        # length with bit 13 set: 5 | 0x2000 = 0x2005
+        # 0x2005 in 14-bit varint: 10_100000 00000101
+        length_bytes = _encode_voice_varint(5 | 0x2000)
+        assert length_bytes in pkt
+
+    def test_no_terminator_by_default(self):
+        opus = b"\x00" * 5
+        pkt = _build_voice_packet(0, opus, last=False)
+        # length=5, no bit 13
+        assert pkt[2] == 5
+
+
+# ---------------------------------------------------------------------------
+# TestPcmScaling
+# ---------------------------------------------------------------------------
+
+
+class TestPcmScaling:
+    def test_unity_volume(self):
+        import struct as _s
+        pcm = _s.pack("<hh", 1000, -1000)
+        result = _scale_pcm(pcm, 1.0)
+        assert result == pcm
+
+    def test_half_volume(self):
+        import struct as _s
+        pcm = _s.pack("<h", 1000)
+        result = _scale_pcm(pcm, 0.5)
+        samples = _s.unpack("<h", result)
+        assert samples[0] == 500
+
+    def test_clamp_positive(self):
+        import struct as _s
+        pcm = _s.pack("<h", 32767)
+        result = _scale_pcm(pcm, 2.0)
+        samples = _s.unpack("<h", result)
+        assert samples[0] == 32767
+
+    def test_clamp_negative(self):
+        import struct as _s
+        pcm = _s.pack("<h", -32768)
+        result = _scale_pcm(pcm, 2.0)
+        samples = _s.unpack("<h", result)
+        assert samples[0] == -32768
+
+    def test_zero_volume(self):
+        import struct as _s
+        pcm = _s.pack("<hh", 32767, -32768)
+        result = _scale_pcm(pcm, 0.0)
+        samples = _s.unpack("<hh", result)
+        assert samples == (0, 0)
+
+    def test_preserves_length(self):
+        pcm = b"\x00" * 1920  # 960 samples
+        result = _scale_pcm(pcm, 0.5)
+        assert len(result) == 1920
+
+
+# ---------------------------------------------------------------------------
+# TestShellQuote
+# ---------------------------------------------------------------------------
+
+
+class TestShellQuote:
+    def test_simple(self):
+        assert _shell_quote("hello") == "'hello'"
+
+    def test_single_quote(self):
+        assert _shell_quote("it's") == "'it'\\''s'"
+
+    def test_url(self):
+        url = "https://youtube.com/watch?v=abc&t=10"
+        quoted = _shell_quote(url)
+        assert quoted.startswith("'")
+        assert quoted.endswith("'")
+
+
+# ---------------------------------------------------------------------------
+# TestMsgUdpTunnel
+# ---------------------------------------------------------------------------
+
+
+class TestMsgUdpTunnel:
+    def test_constant(self):
+        assert MSG_UDPTUNNEL == 1