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_modulator.py

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+"""Tests for PitchShifter."""
+
+import numpy as np
+
+from tuimble.modulator import PitchShifter
+
+SAMPLE_RATE = 48000
+FRAME_SIZE = 960  # 20ms at 48kHz
+
+
+def _sine_pcm(freq: float, n_samples: int = FRAME_SIZE) -> bytes:
+    """Generate a single-frequency int16 PCM frame."""
+    t = np.arange(n_samples) / SAMPLE_RATE
+    samples = (np.sin(2 * np.pi * freq * t) * 16000).astype(np.int16)
+    return samples.tobytes()
+
+
+def _dominant_freq(pcm: bytes) -> float:
+    """Return the dominant frequency in an int16 PCM buffer."""
+    samples = np.frombuffer(pcm, dtype=np.int16).astype(np.float32)
+    fft = np.abs(np.fft.rfft(samples))
+    freqs = np.fft.rfftfreq(len(samples), 1.0 / SAMPLE_RATE)
+    return freqs[np.argmax(fft)]
+
+
+def test_zero_semitones_passthrough():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    assert ps.process(pcm) == pcm
+
+
+def test_pitch_shift_changes_output():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    ps.semitones = 3.0
+    result = ps.process(pcm)
+    assert result != pcm
+
+
+def test_output_length_preserved():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    ps.semitones = 5.0
+    result = ps.process(pcm)
+    assert len(result) == len(pcm)
+
+
+def test_semitones_clamping():
+    ps = PitchShifter()
+    ps.semitones = 20.0
+    assert ps.semitones == 12.0
+    ps.semitones = -20.0
+    assert ps.semitones == -12.0
+    ps.semitones = 5.0
+    assert ps.semitones == 5.0
+
+
+def test_empty_input():
+    ps = PitchShifter()
+    ps.semitones = 3.0
+    assert ps.process(b"") == b""
+    assert ps.process(b"\x00") == b"\x00"
+
+
+def test_pitch_up_frequency_increases():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    ps.semitones = 4.0
+    result = ps.process(pcm)
+    assert _dominant_freq(result) > _dominant_freq(pcm)
+
+
+def test_pitch_down_frequency_decreases():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    ps.semitones = -4.0
+    result = ps.process(pcm)
+    assert _dominant_freq(result) < _dominant_freq(pcm)