refactor: extract _process_subtitle_file domain logic to shared modules

- domain/subtitle_processor.py: parse_subtitle_file, format_time_range,
  speed_up_audio, fit_audio_to_duration (moved from audio_buffer),
  process_subtitle_entries (core TTS loop with cancel/log/progress callbacks)
- domain/audio_buffer.py: add fit_audio_to_duration, ffmpeg_time_stretch
- domain/output_paths.py: add resolve_unique_path (collision-safe filename)
- pyqt/conversion.py: _process_subtitle_file reduced from ~350 to ~100 lines
  by delegating to domain functions; removed 4 unused subtitle parser imports
- +33 tests (8 resolve_unique_path, 14 subtitle_processor, 8 audio_buffer,
  3 format_time_range)
- 1153 tests pass

tests/test_domain_audio_buffer_extras.py

@@ -0,0 +1,59 @@
+"""Tests for domain/audio_buffer.py — fit_audio_to_duration, ffmpeg_time_stretch."""
+
+import numpy as np
+import pytest
+
+from abogen.domain.audio_buffer import fit_audio_to_duration, ffmpeg_time_stretch, SAMPLE_RATE
+
+
+class TestFitAudioToDuration:
+    def test_exact_length(self):
+        audio = np.ones(24000, dtype="float32")
+        result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
+        assert len(result) == 24000
+
+    def test_shorter_pads_with_zeros(self):
+        audio = np.ones(12000, dtype="float32")
+        result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
+        assert len(result) == 24000
+        assert result[0] == 1.0
+        assert result[12000] == 0.0
+
+    def test_longer_trims(self):
+        audio = np.ones(48000, dtype="float32")
+        result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
+        assert len(result) == 24000
+        assert result[-1] == 1.0
+
+    def test_empty_input(self):
+        result = fit_audio_to_duration(np.array([], dtype="float32"), 0.5, SAMPLE_RATE)
+        assert len(result) == 12000
+        assert np.all(result == 0.0)
+
+    def test_output_dtype(self):
+        audio = np.ones(100, dtype="float32")
+        result = fit_audio_to_duration(audio, 0.5, SAMPLE_RATE)
+        assert result.dtype == np.float32
+
+
+class TestFfmpegTimeStretch:
+    def test_no_stretch_below_threshold(self):
+        audio = np.ones(24000, dtype="float32")
+        result = ffmpeg_time_stretch(audio, 0.8, SAMPLE_RATE)
+        np.testing.assert_array_equal(result, audio)
+
+    def test_no_stretch_at_exactly_one(self):
+        audio = np.ones(24000, dtype="float32")
+        result = ffmpeg_time_stretch(audio, 1.0, SAMPLE_RATE)
+        np.testing.assert_array_equal(result, audio)
+
+    def test_empty_audio(self):
+        result = ffmpeg_time_stretch(np.array([], dtype="float32"), 2.0, SAMPLE_RATE)
+        assert len(result) == 0
+
+    def test_stretch_reduces_duration(self):
+        audio = np.random.randn(48000).astype("float32")
+        result = ffmpeg_time_stretch(audio, 2.0, SAMPLE_RATE)
+        assert len(result) < len(audio)
+        assert len(result) > 0
+        assert result.dtype == np.float32