feat: CLI narration --revise and scene-spec title/box subtitles

[jmjava]

Summary

Follow-up after #61:

Change	Detail
CLI revise	docgen narration-generate --revise --revision-notes "…" edits existing narration/*.md in place (same contract as wizard Revise; implies --force)
Title subtitle	Optional title.subtitle (≤80 chars) — second line under the page title; shrinks layout stack budget
Box subtitle	Optional boxes[].subtitle (≤60 chars) — decorative second line; not used for wait_word matching

Usage

docgen narration-generate --segment 01 --revise --revision-notes "Mention the Flask orchestrator earlier"

title:
  text: Architecture
  subtitle: Four pipelines
  font_size: 36
  color: C_WHITE
rows:
  - boxes:
      - label: Flask          # spoken beat anchor
        subtitle: orchestrator  # decorative only

Test plan

• pytest tests/ (314 passed)
• ruff check src/ tests/
• Unit: revise mode passes current narration; CLI revise overwrites
• Unit: title/box subtitle compile + layout budget shrinks with title.subtitle

Note: #60 (story_end / scene-spec av_sync anchors) was rebased onto main after #61 and is ready to merge separately.

  

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