test: add pitch shifting and modulator tests

Covers PitchShifter passthrough, frequency shift direction, output
length preservation, semitone clamping.  AudioPipeline tests verify
pitch property defaults, get/set, clamping, and dequeue integration.

tests/test_audio.py

@@ -2,6 +2,8 @@
 
 import struct
 
+import numpy as np
+
 from tuimble.audio import FRAME_SIZE, AudioPipeline, _apply_gain
 
 
@@ -227,3 +229,43 @@ def test_playback_callback_applies_output_gain():
     ap._playback_callback(outdata, 2, None, None)
     result = struct.unpack("<2h", bytes(outdata))
     assert result == (500, -500)
+
+
+# -- pitch property tests ----------------------------------------------------
+
+
+def test_pitch_default():
+    ap = AudioPipeline()
+    assert ap.pitch == 0.0
+
+
+def test_pitch_set_and_get():
+    ap = AudioPipeline()
+    ap.pitch = 5.0
+    assert ap.pitch == 5.0
+    ap.pitch = -3.0
+    assert ap.pitch == -3.0
+
+
+def test_pitch_clamping():
+    ap = AudioPipeline()
+    ap.pitch = 20.0
+    assert ap.pitch == 12.0
+    ap.pitch = -20.0
+    assert ap.pitch == -12.0
+
+
+def test_pitch_applied_on_dequeue():
+    """Pitch shifting runs in get_capture_frame, not the callback."""
+    ap = AudioPipeline()
+    ap.capturing = True
+    ap.pitch = 4.0
+
+    t = np.arange(FRAME_SIZE) / 48000.0
+    pcm = (np.sin(2 * np.pi * 440.0 * t) * 16000).astype(np.int16).tobytes()
+
+    ap._capture_callback(pcm, FRAME_SIZE, None, None)
+    frame = ap.get_capture_frame()
+    assert frame is not None
+    assert len(frame) == len(pcm)
+    assert frame != pcm