Fabryk v0.1-alpha checkpoint

Phases 1-3 complete. Core infrastructure extracted:

fabryk-core (Phase 1):
- Error types with backtrace support (thiserror)
- File and path utilities
- ConfigProvider trait
- Generic AppState<C>
- PathResolver for env-aware path resolution
- ID utilities (normalize_id, id_from_path)
- 98 unit + 15 doc tests

fabryk-content (Phase 2):
- Frontmatter extraction (generic YAML parsing)
- Markdown parser (heading, paragraph, text extraction)
- Content helpers (section extraction, list parsing, ID normalization)
- 61 unit + 17 doc tests

fabryk-fts (Phase 3):
- Default 14-field Tantivy schema (Amendment §2d)
- SearchBackend trait with SimpleSearch fallback
- TantivySearch with BM25 scoring, weighted multi-field queries
- QueryBuilder with phrase support, query modes, stemming
- DocumentExtractor trait for domain integration
- Indexer, IndexBuilder, content hash freshness
- StopwordFilter with configurable allowlist
- 128 unit + 2 doc tests (with fts-tantivy feature)

Total: 287 unit tests + 34 doc tests = 321 tests passing
All clippy clean, all formatted.

Music-theory integration deferred to separate session.
Graph extraction (Phase 4, highest risk) next.

Ref: Doc 0013 v0.1-alpha checkpoint, Amendment §2e