refactor: extract OutputLayoutService

Extract output path resolution from conversion_planner.py into
application/output_layout_service.py as a standalone service.

Functions:
- resolve_output_layout(request) -> OutputLayout
- resolve_merged_path(layout, request) -> Path
- resolve_chapter_path(layout, request, title, index) -> Path
- should_merge_output(request) -> bool

Planner now imports from output_layout_service instead of inline logic.

abogen/application/conversion_planner.py

@@ -20,17 +20,12 @@ from abogen.application.conversion_models import (
     SegmentPlan,
 )
 from abogen.application.conversion_request import ConversionRequest
+from abogen.application.output_layout_service import resolve_output_layout
 from abogen.domain.chapter_overrides import apply_chapter_overrides
 from abogen.domain.file_type import auto_select_relevant_chapters
 from abogen.domain.intro_outro import resolve_intro, resolve_outro
 from abogen.domain.metadata_extraction import extract_metadata_for_file
 from abogen.domain.metadata_merge import merge_metadata
-from abogen.domain.output_paths import (
-    resolve_output_directory,
-    resolve_project_layout,
-    resolve_unique_path,
-    sanitize_output_stem,
-)
 from abogen.subtitle_utils import split_text_by_voice_markers
 
 
@@ -71,7 +66,7 @@ def build_conversion_plan(request: ConversionRequest) -> ConversionPlan:
     intro, outro = _build_intro_outro(metadata, request)
 
     # 7. Resolve output layout
-    output_layout = _resolve_output_layout(request)
+    output_layout = resolve_output_layout(request)
 
     return ConversionPlan(
         request=request,
@@ -354,47 +349,4 @@ def _build_intro_outro(
     return intro_spec, outro_spec
 
 
-def _resolve_output_layout(request: ConversionRequest) -> OutputLayout:
-    """Resolve output paths for the conversion."""
-    # Determine base output directory
-    if request.save_mode == "custom_folder" and request.output_folder:
-        parent_dir = Path(request.output_folder)
-    elif request.source_path:
-        parent_dir = request.source_path.parent
-    else:
-        parent_dir = Path.cwd()
-
-    # Determine base name
-    if request.original_filename:
-        base_name = sanitize_output_stem(request.original_filename)
-    elif request.source_path:
-        base_name = sanitize_output_stem(request.source_path.stem)
-    else:
-        base_name = "output"
-
-    # Find unique output path
-    allowed_exts = {request.output_format, "srt", "ass", "vtt", "mp4", "m4b"}
-    unique_base = resolve_unique_path(
-        parent_dir, base_name, "", allowed_extensions=allowed_exts
-    )
-
-    # Resolve project layout
-    project_root = None
-    audio_dir = parent_dir
-    subtitle_dir = None
-    metadata_dir = None
-
-    if request.save_as_project:
-        project_root, audio_dir, subtitle_dir, metadata_dir = resolve_project_layout(
-            original_filename=request.original_filename,
-            save_as_project=True,
-            base_dir=parent_dir,
-        )
-
-    return OutputLayout(
-        parent_dir=parent_dir,
-        project_root=project_root,
-        audio_dir=audio_dir,
-        subtitle_dir=subtitle_dir,
-        metadata_dir=metadata_dir,
-    )
+# Output layout resolution is now in application/output_layout_service.py

abogen/application/output_layout_service.py

@@ -0,0 +1,149 @@
+"""Output layout resolution service.
+
+Determines where conversion outputs (audio, subtitles, metadata) should be written.
+Extracted from conversion_planner.py as a standalone service per plan Stage 5.
+
+Responsibilities:
+- Resolve base output directory from save_mode and source_path
+- Determine base filename from original_filename
+- Find unique output path to avoid overwrites
+- Resolve project layout (audio_dir, subtitle_dir, metadata_dir)
+- Force merged output for m4b format
+- Return OutputLayout dataclass
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Optional
+
+from abogen.application.conversion_models import OutputLayout
+from abogen.application.conversion_request import ConversionRequest
+from abogen.domain.output_paths import (
+    resolve_project_layout,
+    resolve_unique_path,
+    sanitize_output_stem,
+)
+
+
+def resolve_output_layout(request: ConversionRequest) -> OutputLayout:
+    """Resolve output paths for a conversion request.
+
+    This is the single entry point for output path resolution,
+    used by both UIs and the conversion service.
+
+    Args:
+        request: Normalized conversion request
+
+    Returns:
+        OutputLayout with resolved paths
+    """
+    # Determine base output directory
+    if request.save_mode == "custom_folder" and request.output_folder:
+        parent_dir = Path(request.output_folder)
+    elif request.source_path:
+        parent_dir = request.source_path.parent
+    else:
+        parent_dir = Path.cwd()
+
+    # Determine base name
+    if request.original_filename:
+        base_name = sanitize_output_stem(request.original_filename)
+    elif request.source_path:
+        base_name = sanitize_output_stem(request.source_path.stem)
+    else:
+        base_name = "output"
+
+    # Find unique output path
+    allowed_exts = {request.output_format, "srt", "ass", "vtt", "mp4", "m4b"}
+    unique_base = resolve_unique_path(
+        parent_dir, base_name, "", allowed_extensions=allowed_exts
+    )
+
+    # Resolve project layout
+    project_root = None
+    audio_dir = parent_dir
+    subtitle_dir = None
+    metadata_dir = None
+
+    if request.save_as_project:
+        project_root, audio_dir, subtitle_dir, metadata_dir = resolve_project_layout(
+            original_filename=request.original_filename,
+            save_as_project=True,
+            base_dir=parent_dir,
+        )
+
+    return OutputLayout(
+        parent_dir=parent_dir,
+        project_root=project_root,
+        audio_dir=audio_dir,
+        subtitle_dir=subtitle_dir,
+        metadata_dir=metadata_dir,
+    )
+
+
+def resolve_merged_path(
+    layout: OutputLayout,
+    request: ConversionRequest,
+) -> Path:
+    """Resolve the merged output audio file path.
+
+    Args:
+        layout: Resolved output layout
+        request: Conversion request
+
+    Returns:
+        Path to the merged output file
+    """
+    base_name = sanitize_output_stem(
+        request.original_filename or "output"
+    )
+    return layout.audio_dir / f"{base_name}.{request.output_format}"
+
+
+def resolve_chapter_path(
+    layout: OutputLayout,
+    request: ConversionRequest,
+    chapter_title: str,
+    chapter_index: int,
+) -> Path:
+    """Resolve the output path for a separate chapter file.
+
+    Args:
+        layout: Resolved output layout
+        request: Conversion request
+        chapter_title: Chapter title for filename
+        chapter_index: Chapter number (1-based)
+
+    Returns:
+        Path to the chapter output file
+    """
+    import re
+
+    slug = re.sub(r'[^\w\s-]', '', chapter_title.lower())
+    slug = re.sub(r'[\s_]+', '_', slug).strip('_')
+    if not slug:
+        slug = f"chapter_{chapter_index}"
+    filename = f"{chapter_index:02d}_{slug}.{request.separate_chapters_format}"
+    return layout.audio_dir / "chapters" / filename
+
+
+def should_merge_output(request: ConversionRequest) -> bool:
+    """Determine if merged output is required.
+
+    Rules:
+    - m4b format always forces merged output
+    - If save_chapters_separately is False, merged is required
+    - Otherwise, use merge_chapters_at_end setting
+
+    Args:
+        request: Conversion request
+
+    Returns:
+        True if merged output should be created
+    """
+    if request.output_format.lower() == "m4b":
+        return True
+    if not request.save_chapters_separately:
+        return True
+    return request.merge_chapters_at_end