Phase 1 implementation: script ingestion + AI extraction pipeline

Complete working pipeline from Fountain script to validated scene JSON:
- Schemas (Pydantic): all 7 layers defined upfront
- Fountain parser + normalizer (Layer 1)
- AI scene extractor with prompt contracts (Layer 2)
- Schema validator + scene-specific semantic validator
- Structured JSON logging per layer/scene execution
- Versioned output writer (never overwrites)
- Retry engine with 4-level failure escalation
- Stop condition evaluator (per-unit + global halts)
- Diff/drift detector for re-run comparison
- CLI entry point with --dry-run, --scene, --test, --force
- 3 test scripts (dialogue-heavy, action-heavy, nonstandard)
- Expected output files for regression testing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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