docs(pdftract-4ij2): add verification note for cycle detection + LRU cache

Implementation already complete. All 9 integration tests pass:
- Self-cycle detection returns PdfNull + STRUCT_CIRCULAR_REF
- 3-cycle (A->B->C->A) detection
- Legitimate objects cache after cycle
- 90%+ cache hit ratio
- LRU eviction at 4097 entries
- Random sequences terminate

Closes pdftract-4ij2.

notes/pdftract-4ij2.md

@@ -0,0 +1,86 @@
+# pdftract-4ij2: Per-thread cycle detection + LRU object cache
+
+## Summary
+
+This bead's implementation was **already complete** in the codebase. The per-thread cycle detection and LRU object cache features have been fully implemented with comprehensive tests.
+
+## Implementation Verification
+
+### Files Modified (already exist)
+
+| File | Purpose |
+|------|---------|
+| `crates/pdftract-core/src/parser/object/cycle.rs` | Per-thread cycle detection with `thread_local! RefCell<HashSet<ObjRef>>` |
+| `crates/pdftract-core/src/parser/object/cache.rs` | LRU cache with `Mutex<LruCache<ObjRef, Arc<PdfObject>>>`, 4096 capacity |
+| `crates/pdftract-core/src/parser/object/types.rs` | ObjRef, PdfObject types |
+| `crates/pdftract-core/tests/test_cycle_detection.rs` | Integration tests |
+
+### Acceptance Criteria Status
+
+| Criterion | Status | Test |
+|----------|--------|------|
+| Self-cycle returns PdfNull + STRUCT_CIRCULAR_REF, no stack overflow | ✅ PASS | `test_self_cycle_returns_null_with_diagnostic` |
+| 3-cycle (A->B->C->A) detection | ✅ PASS | `test_three_cycle_abc_detected` |
+| Legitimate objects cached after cycle detection | ✅ PASS | `test_legitimate_object_after_cycle` |
+| Cache hit ratio >= 90% (1000 accesses of 100 objects) | ✅ PASS | `test_cache_hit_ratio_90_percent` |
+| LRU eviction at 4097 entries (4096 capacity) | ✅ PASS | `test_lru_eviction_4097_entries` |
+| Random resolution sequences terminate | ✅ PASS | `test_random_resolution_sequences_terminate` |
+
+### Implementation Details Verified
+
+#### Per-thread Cycle Detection (`cycle.rs`)
+- ✅ `thread_local! { static RESOLVING: RefCell<HashSet<ObjRef>> ... }`
+- ✅ `ResolutionGuard` RAII guard for automatic cleanup
+- ✅ `is_resolving(obj_ref)` function for cycle checking
+- ✅ Thread-local isolation for rayon parallelism
+
+#### LRU Object Cache (`cache.rs`)
+- ✅ `lru = "0.12"` crate in `Cargo.toml`
+- ✅ `Mutex<LruCache<ObjRef, Arc<PdfObject>>>` with 4096 capacity
+- ✅ `CacheStats { hits: u64, misses: u64 }` tracking
+- ✅ `MAX_RESOLUTION_DEPTH: u16 = 256` depth limiting
+- ✅ Thread-local depth counter with `RESOLUTION_DEPTH`
+- ✅ PdfNull NOT cached (prevents poisoning)
+- ✅ `CacheResolutionGuard` combining cycle + depth tracking
+
+### Test Results
+
+```bash
+$ cargo test --package pdftract-core --test test_cycle_detection
+running 9 tests
+test test_self_cycle_returns_null_with_diagnostic ... ok
+test test_three_cycle_abc_detected ... ok
+test test_legitimate_object_after_cycle ... ok
+test test_cache_hit_ratio_90_percent ... ok
+test test_lru_eviction_4097_entries ... ok
+test test_resolution_depth_limit_256 ... ok
+test test_thread_local_cycle_detection ... ok
+test test_null_not_cached ... ok
+test test_random_resolution_sequences_terminate ... ok
+
+test result: ok. 9 passed; 0 failed; 0 ignored
+```
+
+### Unit Tests (cache.rs)
+
+All 21 unit tests in `cache.rs` pass:
+- Cache hit/miss tracking
+- Hit ratio calculation
+- Null not cached
+- LRU eviction
+- Cycle detection
+- Depth limit enforcement
+- Thread-local independence
+- Guard cleanup on panic
+- peek_lru/is_lru helpers
+
+### Diagnostic Codes Verified
+
+| Code | Emitted When | Message Includes |
+|------|--------------|------------------|
+| `STRUCT_CIRCULAR_REF` | Cycle detected | ObjRef in msg |
+| `STRUCT_DEPTH_EXCEEDED` | Depth > 256 | Limit value in msg |
+
+## Conclusion
+
+**The implementation is complete and all acceptance criteria PASS.** No code changes were required for this bead—the feature was already implemented.