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_subtitle_processor.py

@@ -0,0 +1,169 @@
+"""Tests for domain/subtitle_processor.py — parse_subtitle_file, format_time_range, process_subtitle_entries."""
+
+import os
+import tempfile
+from dataclasses import dataclass
+from typing import Optional
+from unittest.mock import MagicMock
+
+import numpy as np
+import pytest
+
+from abogen.domain.subtitle_processor import (
+    parse_subtitle_file,
+    format_time_range,
+    speed_up_audio,
+    process_subtitle_entries,
+)
+
+
+# --- format_time_range tests ---
+
+class TestFormatTimeRange:
+    def test_basic_range(self):
+        result = format_time_range(0.0, 5.0)
+        assert result == "00:00:00 - 00:00:05"
+
+    def test_with_milliseconds(self):
+        result = format_time_range(1.5, 3.123)
+        assert "00:00:01,500" in result
+        assert "00:00:03,123" in result
+
+    def test_auto_end(self):
+        result = format_time_range(10.0, 15.0, is_auto_end=True)
+        assert result == "00:00:10 - AUTO"
+
+    def test_none_end(self):
+        result = format_time_range(5.0, None)
+        assert result == "00:00:05 - AUTO"
+
+    def test_hours(self):
+        result = format_time_range(3661.0, 3665.0)
+        assert result == "01:01:01 - 01:01:05"
+
+
+# --- parse_subtitle_file tests ---
+
+class TestParseSubtitleFile:
+    def test_parse_srt(self, tmp_path):
+        srt = tmp_path / "test.srt"
+        srt.write_text(
+            "1\n00:00:01,000 --> 00:00:03,000\nHello\n\n"
+            "2\n00:00:04,000 --> 00:00:06,000\nWorld\n",
+            encoding="utf-8",
+        )
+        result = parse_subtitle_file(str(srt))
+        assert len(result) == 2
+        assert result[0][2] == "Hello"
+        assert result[1][2] == "World"
+
+    def test_parse_vtt(self, tmp_path):
+        vtt = tmp_path / "test.vtt"
+        vtt.write_text(
+            "WEBVTT\n\n00:00:01.000 --> 00:00:03.000\nHello\n\n"
+            "00:00:04.000 --> 00:00:06.000\nWorld\n",
+            encoding="utf-8",
+        )
+        result = parse_subtitle_file(str(vtt))
+        assert len(result) == 2
+
+    def test_parse_timestamp_text(self, tmp_path):
+        ts = tmp_path / "test.txt"
+        ts.write_text(
+            "[00:00:01] Hello\n[00:00:04] World\n",
+            encoding="utf-8",
+        )
+        result = parse_subtitle_file(str(ts), is_timestamp_text=True)
+        assert len(result) >= 1
+
+
+# --- speed_up_audio tests ---
+
+class TestSpeedUpAudio:
+    def test_no_change_below_threshold(self):
+        audio = np.ones(24000, dtype="float32")
+        result = speed_up_audio(audio, 0.8, method="ffmpeg")
+        np.testing.assert_array_equal(result, audio)
+
+    def test_empty_audio(self):
+        result = speed_up_audio(np.array([], dtype="float32"), 2.0, method="ffmpeg")
+        assert len(result) == 0
+
+
+# --- process_subtitle_entries tests ---
+
+@dataclass
+class FakeResult:
+    audio: np.ndarray
+
+
+def fake_backend(text, voice=None, speed=1.0, split_pattern=None):
+    length = int(len(text) * 2400 * speed)
+    audio = np.random.randn(length).astype("float32") * 0.1
+    return [FakeResult(audio=audio)]
+
+
+class TestProcessSubtitleEntries:
+    def test_empty_subtitles(self):
+        result = process_subtitle_entries(
+            [], backend=fake_backend, voice=None
+        )
+        assert len(result) == 0
+
+    def test_single_entry(self):
+        subtitles = [(0.0, 3.0, "Hello")]
+        result = process_subtitle_entries(
+            subtitles, backend=fake_backend, voice=None
+        )
+        assert len(result) > 0
+        assert result.dtype == np.float32
+
+    def test_cancel_check(self):
+        subtitles = [(0.0, 5.0, "Hello"), (5.0, 10.0, "World")]
+        counter = [0]
+
+        def cancel():
+            counter[0] += 1
+            return counter[0] > 1
+
+        result = process_subtitle_entries(
+            subtitles, backend=fake_backend, voice=None,
+            cancel_check=cancel,
+        )
+        # Buffer is pre-allocated but only first entry processed before cancel
+        assert result is not None
+        # Second entry should not have been mixed in (no audio at 5-10s)
+        assert np.max(np.abs(result[int(5.0 * 24000):])) == 0.0
+
+    def test_log_callback_called(self):
+        subtitles = [(0.0, 3.0, "Hello")]
+        logs = []
+        process_subtitle_entries(
+            subtitles, backend=fake_backend, voice=None,
+            log_callback=logs.append,
+        )
+        assert len(logs) >= 1
+        assert "Hello" in logs[0]
+
+    def test_progress_callback_called(self):
+        subtitles = [(0.0, 3.0, "Hello")]
+        progress = []
+        process_subtitle_entries(
+            subtitles, backend=fake_backend, voice=None,
+            progress_callback=lambda p, e: progress.append((p, e)),
+        )
+        assert len(progress) == 1
+        assert progress[0][0] == 99
+
+    def test_multiple_entries_mixed(self):
+        subtitles = [
+            (0.0, 2.0, "First"),
+            (2.0, 4.0, "Second"),
+            (4.0, 6.0, "Third"),
+        ]
+        result = process_subtitle_entries(
+            subtitles, backend=fake_backend, voice=None
+        )
+        assert len(result) > 0
+        # Buffer should be at least as long as the last subtitle end
+        assert len(result) >= int(6.0 * 24000)