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_resolve_unique_path.py

@@ -0,0 +1,51 @@
+"""Tests for domain/output_paths.py — resolve_unique_path."""
+
+import os
+import tempfile
+
+from abogen.domain.output_paths import resolve_unique_path
+
+
+class TestResolveUniquePath:
+    def test_no_collision(self, tmp_path):
+        result = resolve_unique_path(str(tmp_path), "chapter", "srt")
+        assert result == os.path.join(str(tmp_path), "chapter")
+        assert not os.path.exists(result)
+
+    def test_collision_appends_counter(self, tmp_path):
+        (tmp_path / "chapter.srt").touch()
+        result = resolve_unique_path(str(tmp_path), "chapter", "srt", {"srt"})
+        assert result == os.path.join(str(tmp_path), "chapter_2")
+
+    def test_multiple_collisions(self, tmp_path):
+        (tmp_path / "chapter.srt").touch()
+        (tmp_path / "chapter_2.srt").touch()
+        (tmp_path / "chapter_3.srt").touch()
+        result = resolve_unique_path(str(tmp_path), "chapter", "srt", {"srt"})
+        assert result == os.path.join(str(tmp_path), "chapter_4")
+
+    def test_no_allowed_extensions_skips_files(self, tmp_path):
+        (tmp_path / "chapter.txt").touch()
+        result = resolve_unique_path(str(tmp_path), "chapter", "srt")
+        assert result == os.path.join(str(tmp_path), "chapter")
+
+    def test_sanitizes_name(self, tmp_path):
+        # On Windows, ":" is illegal; on Linux it's allowed.
+        # Just verify the function doesn't crash and returns a valid path.
+        result = resolve_unique_path(str(tmp_path), "My Chapter: Part 1", "srt")
+        assert os.path.dirname(result) == str(tmp_path)
+
+    def test_directory_collision(self, tmp_path):
+        (tmp_path / "chapter").mkdir()
+        result = resolve_unique_path(str(tmp_path), "chapter", "srt")
+        assert result == os.path.join(str(tmp_path), "chapter_2")
+
+    def test_case_insensitive_extension(self, tmp_path):
+        (tmp_path / "chapter.SRT").touch()
+        result = resolve_unique_path(str(tmp_path), "chapter", "srt", {"srt"})
+        assert result == os.path.join(str(tmp_path), "chapter_2")
+
+    def test_unrelated_extensions_no_collision(self, tmp_path):
+        (tmp_path / "chapter.mp3").touch()
+        result = resolve_unique_path(str(tmp_path), "chapter", "srt", {"srt"})
+        assert result == os.path.join(str(tmp_path), "chapter")