refactor: create domain/conversion_pipeline.py with shared TTS emission loop

- domain/conversion_pipeline.py: emit_text_segments() — generator yielding
  SegmentResult for each TTS segment; emit_text_to_sinks() — convenience
  wrapper handling audio writing + token accumulation + subtitle flushing
- Both WebUI and PyQt can call these instead of reimplementing the TTS loop
- Caller provides backend, voice, speed, split_pattern; domain handles
  normalization, TTS invocation, token extraction
- +7 tests (segment yielding, empty audio skip, chunk_start, tokens, fallback)
- 1185 tests pass

abogen/domain/conversion_pipeline.py

@@ -0,0 +1,208 @@
+"""Shared TTS emission pipeline.
+
+Provides the core TTS emission loop used by both WebUI and PyQt conversion runners.
+The caller handles audio I/O, progress reporting, and subtitle writing.
+"""
+
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass, field
+from typing import Any, Callable, Dict, Iterator, List, Optional
+
+import numpy as np
+
+from abogen.domain.audio_helpers import to_float32
+from abogen.domain.normalization import prepare_text_for_tts
+from abogen.domain.tokens import FakeToken
+from abogen.domain.audio_buffer import SAMPLE_RATE
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class SegmentResult:
+    """One TTS segment emitted by the pipeline."""
+    graphemes: str
+    audio: np.ndarray
+    duration: float
+    chunk_start: float
+    tokens: List[Dict[str, Any]] = field(default_factory=list)
+
+
+def emit_text_segments(
+    text: str,
+    *,
+    backend: Any,
+    voice: Any,
+    speed: float,
+    split_pattern: str,
+    current_time: float = 0.0,
+    # normalization
+    heteronym_rules: Any = None,
+    pronunciation_rules: Any = None,
+    normalization_overrides: Any = None,
+    usage_counter: Optional[Dict[str, int]] = None,
+) -> Iterator[SegmentResult]:
+    """Normalize text and yield SegmentResults from the TTS backend.
+
+    This is the innermost TTS emission loop shared by both UIs. It handles:
+    1. Text normalization (heteronym + pronunciation rules)
+    2. TTS backend invocation
+    3. Segment iteration with token extraction
+
+    The caller is responsible for:
+    - Writing audio to sinks
+    - Accumulating tokens for subtitle processing
+    - Progress tracking and cancellation
+    - Error handling
+
+    Args:
+        text: Raw text to synthesize.
+        backend: TTS pipeline callable (kokoro or supertonic).
+        voice: Resolved voice for TTS.
+        speed: TTS speed multiplier.
+        split_pattern: Regex pattern for sentence splitting.
+        current_time: Current position in the audio timeline (seconds).
+        heteronym_rules: Compiled heteronym rules.
+        pronunciation_rules: Compiled pronunciation rules.
+        normalization_overrides: User normalization overrides.
+        usage_counter: Counter for normalization statistics.
+
+    Yields:
+        SegmentResult for each non-empty TTS segment.
+    """
+    source_text = str(text or "")
+    normalized = prepare_text_for_tts(
+        source_text,
+        heteronym_rules=heteronym_rules,
+        pronunciation_rules=pronunciation_rules,
+        normalization_overrides=normalization_overrides,
+        usage_counter=usage_counter,
+    )
+
+    segment_iter = backend(
+        normalized,
+        voice=voice,
+        speed=speed,
+        split_pattern=split_pattern,
+    )
+
+    chunk_start = current_time
+
+    for segment in segment_iter:
+        graphemes_raw = getattr(segment, "graphemes", "") or ""
+        graphemes = graphemes_raw.strip()
+
+        audio = to_float32(getattr(segment, "audio", None))
+        if audio.size == 0:
+            continue
+
+        duration = len(audio) / SAMPLE_RATE
+
+        # Extract tokens with timestamps
+        tokens_list = getattr(segment, "tokens", [])
+        if not tokens_list and graphemes:
+            tokens_list = [FakeToken(graphemes, 0, duration)]
+
+        tokens = [
+            {
+                "start": chunk_start + (tok.start_ts or 0),
+                "end": chunk_start + (tok.end_ts or 0),
+                "text": tok.text,
+                "whitespace": tok.whitespace,
+            }
+            for tok in tokens_list
+        ]
+
+        yield SegmentResult(
+            graphemes=graphemes,
+            audio=audio,
+            duration=duration,
+            chunk_start=chunk_start,
+            tokens=tokens,
+        )
+
+        chunk_start += duration
+
+
+def emit_text_to_sinks(
+    text: str,
+    *,
+    backend: Any,
+    voice: Any,
+    speed: float,
+    split_pattern: str,
+    current_time: float = 0.0,
+    # sinks
+    audio_sink: Any = None,
+    chapter_sink: Any = None,
+    # subtitle
+    subtitle_writer: Any = None,
+    subtitle_mode: str = "Disabled",
+    subtitle_lang: str = "a",
+    max_subtitle_words: int = 50,
+    use_spacy_segmentation: bool = True,
+    # normalization
+    heteronym_rules: Any = None,
+    pronunciation_rules: Any = None,
+    normalization_overrides: Any = None,
+    usage_counter: Optional[Dict[str, int]] = None,
+) -> tuple[int, float, List[Dict[str, Any]]]:
+    """Emit TTS audio for text, writing to sinks and collecting subtitle tokens.
+
+    Convenience wrapper around emit_text_segments() that handles audio writing
+    and token accumulation. Returns stats for the caller to update progress.
+
+    Returns:
+        Tuple of (segments_emitted, new_current_time, accumulated_tokens).
+    """
+    from abogen.domain.subtitle_generation import process_subtitle_tokens
+
+    segments_emitted = 0
+    accumulated_tokens: List[Dict[str, Any]] = []
+
+    for seg in emit_text_segments(
+        text,
+        backend=backend,
+        voice=voice,
+        speed=speed,
+        split_pattern=split_pattern,
+        current_time=current_time,
+        heteronym_rules=heteronym_rules,
+        pronunciation_rules=pronunciation_rules,
+        normalization_overrides=normalization_overrides,
+        usage_counter=usage_counter,
+    ):
+        segments_emitted += 1
+
+        # Write audio
+        if chapter_sink:
+            chapter_sink.write(seg.audio)
+        if audio_sink:
+            audio_sink.write(seg.audio)
+
+        # Collect tokens
+        accumulated_tokens.extend(seg.tokens)
+
+    # Flush subtitle tokens
+    if subtitle_writer and accumulated_tokens:
+        _use_spacy = subtitle_mode not in ("Disabled", "Line")
+        new_entries: List[tuple] = []
+        process_subtitle_tokens(
+            accumulated_tokens,
+            new_entries,
+            max_subtitle_words,
+            subtitle_mode,
+            subtitle_lang,
+            use_spacy_segmentation=_use_spacy,
+            fallback_end_time=current_time + sum(t["end"] - t["start"] for t in accumulated_tokens if accumulated_tokens),
+        )
+        for start, end, text_entry in new_entries:
+            subtitle_writer.write_entry(start=start, end=end, text=text_entry)
+
+    new_time = current_time
+    if accumulated_tokens:
+        new_time = max(t["end"] for t in accumulated_tokens)
+
+    return segments_emitted, new_time, accumulated_tokens