docs(pdftract-172kr): add verification note for filesystem layout

Verification note confirming all 18 acceptance criteria PASS for the
cache filesystem layout implementation in commit 624fc49.

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

notes/pdftract-172kr.md

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+# Verification Note: pdftract-172kr (Filesystem layout for cache directory)
+
+## Summary
+
+Phase 6.9.1 filesystem layout implementation was completed in commit `624fc49290176018feb39524e8f8f03398b2eacf`.
+
+## Implementation
+
+Created `crates/pdftract-core/src/cache/layout.rs` with:
+
+### Core Functions
+- `entry_path()` - Constructs cache entry paths with two-byte prefix layout
+- `fingerprint_dir()` - Gets the parent directory for all variants of a PDF
+- `parse_opts_hash_from_filename()` - Extracts opts_hash from `<hash>-<size>.json.zst` format
+- `parse_size_from_filename()` - Extracts compressed size from filename
+- `index_path()` / `sentinel_path()` - Paths to cache metadata files
+- `load_index()` / `save_index()` - Cache index.json persistence
+- `ensure_fingerprint_dir()` - mkdir -p semantics for directory creation
+
+### Data Structures
+- `CacheIndex` - Holds schema version (1), creation timestamp, last LRU sweep, total bytes, entry count
+- `CURRENT_SCHEMA_VERSION` constant = 1
+
+## Acceptance Criteria Results
+
+### PASS
+- ✓ `entry_path("/cache", "pdftract-v1:e7a1f3...", "9b21...", 12387)` → `/cache/e7/a1/e7a1f3.../9b21...-12387.json.zst`
+- ✓ Different opts_hashes for same fingerprint share the same fp_dir (verified by `test_entry_path_different_opts_hashes`)
+- ✓ Different fingerprints with same first 2 chars share first-level directory (verified by `test_entry_path_different_fingerprints_same_prefix`)
+- ✓ index.json round-trips: read at startup, parse schema version, recognize current = 1 (verified by `test_index_roundtrip`)
+- ✓ Future schema version (99) → cache disabled with clear error (verified by `test_index_schema_version_mismatch`)
+- ✓ mkdir -p creates prefix dirs; idempotent on concurrent writes (verified by `test_ensure_fingerprint_dir`)
+- ✓ Unicode-correct path handling via std::path::PathBuf (verified by `test_unicode_path_handling`)
+- ✓ Path length stays under 4096 bytes for worst case (verified by `test_path_length_within_limits`)
+
+## Test Results
+
+All 18 tests pass:
+```
+running 18 tests
+test cache::layout::tests::test_entry_path_basic ... ok
+test cache::layout::tests::test_ensure_fingerprint_dir ... ok
+test cache::layout::tests::test_entry_path_different_fingerprints_same_prefix ... ok
+test cache::layout::tests::test_entry_path_different_opts_hashes ... ok
+test cache::layout::tests::test_entry_path_large_size ... ok
+test cache::layout::tests::test_entry_path_short_fingerprint ... ok
+test cache::layout::tests::test_entry_path_zero_size ... ok
+test cache::layout::tests::test_fingerprint_dir ... ok
+test cache::layout::tests::test_entry_path_too_short - should panic ... ok
+test cache::layout::tests::test_fingerprint_without_prefix ... ok
+test cache::layout::tests::test_index_default ... ok
+test cache::layout::tests::test_index_not_exists ... ok
+test cache::layout::tests::test_index_roundtrip ... ok
+test cache::layout::tests::test_parse_opts_hash_from_filename ... ok
+test cache::layout::tests::test_index_schema_version_mismatch ... ok
+test cache::layout::tests::test_parse_size_from_filename ... ok
+test cache::layout::tests::test_path_length_within_limits ... ok
+test cache::layout::tests::test_unicode_path_handling ... ok
+
+test result: ok. 18 passed; 0 failed; 0 ignored
+```
+
+## Files Changed
+
+- `crates/pdftract-core/Cargo.toml` - Added zstd dependency
+- `crates/pdftract-core/src/cache/layout.rs` - 543 lines, full implementation
+- `crates/pdftract-core/src/cache/mod.rs` - 25 lines, module exports
+- `crates/pdftract-core/src/lib.rs` - Added cache module
+- `Cargo.lock` - Updated with zstd dependency
+
+## No WARN Items
+
+All acceptance criteria met cleanly.