feat: Implement speaker analysis and EPUB 3 export functionality

- Added speaker analysis module to infer speaker identities from text chunks.
- Introduced SpeakerGuess and SpeakerAnalysis data classes for managing speaker data.
- Developed functions for analyzing speaker occurrences and confidence levels.
- Created EPUB 3 exporter to generate EPUB packages with synchronized narration and media overlays.
- Implemented configurable chunking options for TTS synthesis and EPUB alignment.
- Enhanced JavaScript for speaker preview functionality in the web interface.
- Added comprehensive tests for chunking and EPUB exporting features.
- Documented upgrade plan for transitioning to EPUB 3 with multi-speaker support.

tests/test_chunk_helpers.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+from abogen.web.conversion_runner import _chunk_voice_spec, _group_chunks_by_chapter
+
+
+def test_group_chunks_by_chapter_orders_and_groups() -> None:
+    chunks = [
+        {"chapter_index": "0", "chunk_index": "5", "text": "tail"},
+        {"chapter_index": 0, "chunk_index": 1, "text": "body"},
+        {"chapter_index": 1, "chunk_index": 0, "text": "next"},
+    ]
+
+    grouped = _group_chunks_by_chapter(chunks)
+
+    assert [entry["text"] for entry in grouped[0]] == ["body", "tail"]
+    assert grouped[1][0]["text"] == "next"
+
+
+def test_chunk_voice_spec_prefers_chunk_overrides() -> None:
+    job = SimpleNamespace(voice="base_voice", speakers={})
+    chunk = {"voice": "override_voice", "speaker_id": "narrator"}
+
+    assert _chunk_voice_spec(job, chunk, "fallback") == "override_voice"
+
+
+def test_chunk_voice_spec_falls_back_to_speaker_voice() -> None:
+    job = SimpleNamespace(voice="base_voice", speakers={"narrator": {"voice": "speaker_voice"}})
+    chunk = {"speaker_id": "narrator"}
+
+    assert _chunk_voice_spec(job, chunk, "fallback") == "speaker_voice"
+
+
+def test_chunk_voice_spec_uses_fallback_when_no_overrides() -> None:
+    job = SimpleNamespace(voice="base_voice", speakers={})
+    chunk = {"speaker_id": "unknown"}
+
+    assert _chunk_voice_spec(job, chunk, "fallback") == "fallback"