feat: smooth volume ramping over 200ms in audio streaming

Volume changes now ramp linearly per-sample via _scale_pcm_ramp instead
of jumping abruptly. Each frame steps _cur_vol toward target by at most
0.1, giving ~200ms for a full 0-to-1 sweep. Fast path unchanged when
volume is stable.

tests/test_mumble.py

@@ -2,13 +2,14 @@
 
 import asyncio
 import struct
-from unittest.mock import patch, MagicMock
+from unittest.mock import patch
 
 from derp.mumble import (
     MumbleBot,
     MumbleMessage,
     _escape_html,
     _scale_pcm,
+    _scale_pcm_ramp,
     _shell_quote,
     _strip_html,
 )
@@ -583,3 +584,64 @@ class TestShellQuote:
         quoted = _shell_quote(url)
         assert quoted.startswith("'")
         assert quoted.endswith("'")
+
+
+# ---------------------------------------------------------------------------
+# TestPcmRamping
+# ---------------------------------------------------------------------------
+
+
+class TestPcmRamping:
+    def test_flat_when_equal(self):
+        """When vol_start == vol_end, behaves like _scale_pcm."""
+        pcm = struct.pack("<hh", 1000, -1000)
+        result = _scale_pcm_ramp(pcm, 0.5, 0.5)
+        expected = _scale_pcm(pcm, 0.5)
+        assert result == expected
+
+    def test_linear_interpolation(self):
+        """Volume ramps linearly from start to end across samples."""
+        pcm = struct.pack("<hhhh", 10000, 10000, 10000, 10000)
+        result = _scale_pcm_ramp(pcm, 0.0, 1.0)
+        samples = struct.unpack("<hhhh", result)
+        # At i=0: vol=0.0, i=1: vol=0.25, i=2: vol=0.5, i=3: vol=0.75
+        assert samples[0] == 0
+        assert samples[1] == 2500
+        assert samples[2] == 5000
+        assert samples[3] == 7500
+
+    def test_clamp_positive(self):
+        """Ramping up with loud samples clamps to 32767."""
+        pcm = struct.pack("<h", 32767)
+        result = _scale_pcm_ramp(pcm, 2.0, 2.0)
+        samples = struct.unpack("<h", result)
+        assert samples[0] == 32767
+
+    def test_clamp_negative(self):
+        """Ramping up with negative samples clamps to -32768."""
+        pcm = struct.pack("<h", -32768)
+        result = _scale_pcm_ramp(pcm, 2.0, 2.0)
+        samples = struct.unpack("<h", result)
+        assert samples[0] == -32768
+
+    def test_preserves_length(self):
+        """Output length equals input length."""
+        pcm = b"\x00" * 1920
+        result = _scale_pcm_ramp(pcm, 0.0, 1.0)
+        assert len(result) == 1920
+
+    def test_empty_data(self):
+        """Empty input returns empty output."""
+        result = _scale_pcm_ramp(b"", 0.0, 1.0)
+        assert result == b""
+
+    def test_reverse_direction(self):
+        """Volume ramps down from start to end."""
+        pcm = struct.pack("<hhhh", 10000, 10000, 10000, 10000)
+        result = _scale_pcm_ramp(pcm, 1.0, 0.0)
+        samples = struct.unpack("<hhhh", result)
+        # At i=0: vol=1.0, i=1: vol=0.75, i=2: vol=0.5, i=3: vol=0.25
+        assert samples[0] == 10000
+        assert samples[1] == 7500
+        assert samples[2] == 5000
+        assert samples[3] == 2500