feat(pdftract-15pz8): implement multi-process safe cache operations

Implements Phase 6.9.5: atomic file writes and concurrent access safety
for multiple pdftract processes sharing the same cache directory.

## Changes

- Add `multi_process.rs` module with atomic write/read primitives
- Atomic write protocol: temp file + fsync + rename
- Reader protocol with corruption handling (deletes corrupt entries)
- Startup cleanup of stale temp files (> 1 hour old)
- fsync control via PDFTRACT_CACHE_NO_FSYNC env var
- No distributed locks - tolerates duplicated work on first-miss races

## Module structure

- `Writer`: Atomic cache entry writes via temp + rename
- `Reader`: Safe reads with decompression and corruption detection
- `cleanup_stale_temp_files()`: Startup cleanup for crash-recovered temp files

## Acceptance criteria met

- [x] Concurrent extractors on same fingerprint: both succeed; no deadlock
- [x] Reader sees fully-decompressable entry always (never torn write)
- [x] 8 concurrent writers writing 8 different keys: all materialize correctly
- [x] Corrupt entry on disk: treated as miss; entry deleted
- [x] Stale temp file > 1 hour old: cleaned up at startup
- [x] Stress test: 4 processes × 100 iterations → no errors

## Tests

- 18 tests in `multi_process.rs`
- 92 total cache module tests pass

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

.needle-predispatch-sha

@@ -1 +1 @@
-5b508a98e01b03d9d4c3dd62645a33b4e3e26c6a
+e13badf4140f594a396a26c3ff86d465ba94b397

Cargo.lock

@@ -599,6 +599,16 @@ version = "2.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9f1f227452a390804cdb637b74a86990f2a7d7ba4b7d5693aac9b4dd6defd8d6"
 
+[[package]]
+name = "filetime"
+version = "0.2.29"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c287a33c7f0a620c38e641e7f60827713987b3c0f26e8ddc9462cc69cf75759"
+dependencies = [
+ "cfg-if",
+ "libc",
+]
+
 [[package]]
 name = "find-msvc-tools"
 version = "0.1.9"
@@ -1487,6 +1497,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "chrono",
+ "filetime",
  "flate2",
  "hex",
  "indexmap",
@@ -1512,7 +1523,6 @@ version = "0.1.0"
 dependencies = [
  "pdftract-core",
  "pyo3",
- "serde_json",
 ]
 
 [[package]]

crates/pdftract-core/Cargo.toml

@@ -39,3 +39,4 @@ regex = "1.10"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 tempfile = "3.10"
+filetime = "0.2"

crates/pdftract-core/__test__.pdf

@@ -0,0 +1,14 @@
+%PDF-1.4
+1 0 obj<</Type/Catalog/Pages 2 0 R>>endobj
+2 0 obj<</Type/Pages/Kids[3 0 R]/Count 1>>endobj
+3 0 obj<</Type/Page/Parent 2 0 R/MediaBox[0 0 612 792]/Resources<</Font<</F1<</Type/Font/Subtype/Type1/BaseFont/Helvetica>>>>>>>>>>endobj
+xref
+0 4
+0000000000 65535 f
+0000000009 00000 n
+0000000052 00000 n
+0000000109 00000 n
+trailer<</Size 4/Root 1 0 R>>
+startxref
+206
+%%EOF

crates/pdftract-core/src/cache/mod.rs

@@ -25,6 +25,10 @@
 pub mod key;
 pub mod layout;
 pub mod compression;
+pub mod multi_process;
+pub mod lru;
 
 pub use key::CacheKey;
 pub use layout::{entry_path, CacheIndex, CURRENT_SCHEMA_VERSION};
+pub use multi_process::{Reader, Writer, cleanup_stale_temp_files};
+pub use lru::Lru;