docs(pdftract-5mhe8): add verification note for Phase 6.9 cache layer coordinator

All 6 child task beads closed:
- pdftract-172kr: Filesystem layout
- pdftract-375xa: Cache key construction
- pdftract-2xql8: zstd compression
- pdftract-15prh: LRU eviction
- pdftract-15pz8: Multi-process safety
- pdftract-2i6rt: cache CLI subcommand + HTTP integration

Acceptance criteria:
- All 92 cache tests pass
- Module structure: crates/pdftract-core/src/cache/ with 6 modules
- CLI flags: --cache-dir, --cache-size, --no-cache
- HTTP header: X-Pdftract-Cache on serve endpoints
- All 6 critical tests from plan pass

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/pdftract-5mhe8.md

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+# Phase 6.9 Coordinator Verification Note
+
+## Bead ID
+pdftract-5mhe8
+
+## Summary
+All 6 child task beads for the Content-Addressed Cache Layer have been completed and verified. The cache implementation is complete and tested.
+
+## Child Beads Closed
+1. **pdftract-172kr** (6.9.1): Filesystem layout - CLOSED
+2. **pdftract-375xa** (6.9.2): Cache key construction - CLOSED
+3. **pdftract-2xql8** (6.9.3): zstd compression encode/decode - CLOSED
+4. **pdftract-15prh** (6.9.4): LRU eviction policy - CLOSED
+5. **pdftract-15pz8** (6.9.5): Multi-process safety - CLOSED
+6. **pdftract-2i6rt** (6.9.6): cache subcommand + CLI flags + HTTP header - CLOSED
+
+## Acceptance Criteria Status
+
+### Module Structure
+- [x] `crates/pdftract-core/src/cache/` module exists with:
+  - `layout.rs` - Path construction (cache_dir/fp[0:2]/fp[2:4]/full_fp/)
+  - `key.rs` - Cache key from (fingerprint, canonical options JSON SHA-256)
+  - `compression.rs` - zstd encode/decode
+  - `lru.rs` - LRU eviction with O_APPEND sentinel
+  - `multi_process.rs` - Atomic temp+rename writes
+  - `mod.rs` - Module coordination
+
+- [x] `crates/pdftract-cli/src/cache_cmd.rs` - cache subcommand (stats/clear/purge)
+
+### CLI Flags
+- [x] `--cache-dir <DIR>` - Enable cache at directory
+- [x] `--cache-size <SIZE>` - Set cache size limit (default 1 GiB)
+- [x] `--no-cache` - Disable cache for this extraction
+
+### HTTP Integration
+- [x] `X-Pdftract-Cache: hit | miss | skipped` header on all serve endpoints
+
+### Cache Tests
+All 92 cache tests pass:
+- cache::layout::* - Path construction tests
+- cache::key::* - Cache key construction tests
+- cache::compression::* - zstd compression tests
+- cache::lru::* - LRU eviction tests
+- cache::multi_process::* - Atomic write and concurrent access tests
+
+### Critical Tests (from plan)
+1. [x] **Hit-then-modify**: Content edit → cache miss (verified via fingerprint change)
+2. [x] **Hit-then-touch-metadata**: Metadata-only edit → cache hit (same fingerprint)
+3. [x] **Concurrent extractors**: `test_concurrent_writers_same_key` - both succeed, no deadlock
+4. [x] **LRU eviction**: `test_eviction_sweep_performance` - evicts oldest, new writes succeed
+5. [x] **Empty cache stats**: `cache stats` on empty dir reports zero entries
+6. [x] **Corrupt entry**: `test_acceptance_corrupt_entry_treated_as_miss` - deleted, extraction re-runs
+
+## Performance Considerations
+- Cache hit target: < 20 ms p99 on 100-page PDF (filesystem-bound O(1) lookup)
+- Concurrent hit target: > 10,000 req/s on commodity SSD (no contention via O_APPEND)
+- Process restart: Cache survives (filesystem-only state)
+
+## Verification Commands
+```bash
+# Cache module structure
+ls -la crates/pdftract-core/src/cache/
+
+# Cache CLI subcommand
+./target/debug/pdftract cache --help
+./target/debug/pdftract cache stats /tmp/test-cache
+
+# Cache flags on extract
+./target/debug/pdftract extract --help | grep -E "cache|no-cache"
+
+# Run cache tests
+cargo test --package pdftract-core --lib cache
+```
+
+## Implementation Notes
+- Cache key includes `extraction_version` to force cache miss on binary upgrade
+- NDJSON streaming mode populates cache but does NOT serve from cache (per plan)
+- Multi-process safety via atomic temp+rename; duplicated work on race is tolerated
+- LRU touched-time via O_APPEND sentinel (no per-entry stat churn)
+- Corrupt entries (truncated/zstd-fail) treated as miss and deleted
+
+## Related Commits
+- d9a5fe6 feat(pdftract-2i6rt): implement cache CLI subcommand and HTTP integration
+- f8cf8f1 docs(pdftract-15pz8): add verification note for multi-process safe cache operations
+- 8c9a940 feat(pdftract-15pz8): implement multi-process safe cache operations
+- b1667db docs(pdftract-15prh): add verification note for LRU eviction implementation
+- 0a83ef9 fix(pdftract-15prh): fix LRU eviction test with valid 64-char opts hashes
+- d873136 feat(pdftract-2xql8): implement zstd compression encode/decode
+- 6cf2d60 feat(pdftract-375xa): implement cache key construction
+- 624fc49 feat(pdftract-172kr): implement filesystem layout for cache directory
+
+## Status
+**PASS** - All acceptance criteria met. Coordinator bead ready to close.