feat(validate): story_end check, scene-spec av_sync anchors, layout budgets

[jmjava]

Summary

Continues the beat-sync checklist after #58 / #56:

#	Change
5	story_end validation — last paced scene-spec reveal vs audio/transcript end; hard-fail in --pre-push when idle exceeds both max_early_sec (40s) and max_early_ratio (0.45)
6	av_sync prefers paced scene-spec labels as OCR anchors (falls back to long transcript words only when no specs)
9	Scene-spec LLM prompt includes computed layout_stack_budget numbers and horizontal safe width

Rebased onto main after #61.

Config

validation:
  story_end:
    enabled: true
    max_early_sec: 40.0
    max_early_ratio: 0.45
  av_sync:
    prefer_scene_spec_labels: true

Test plan

• pytest tests/
• ruff check src/ tests/
• Unit: last_paced_reveal_time / pace:none ignored
• Unit: story_end early-fail + hard pre-push
• Unit: av_sync anchors from scene-spec labels (not unrelated long words)
• Unit: user message includes computed layout budgets