feat(native_rust): implement BLAKE3 dataset_hash and priority queue_index
Implements two production-ready Rust native libraries:
## dataset_hash (BLAKE3-based hashing)
- FFI exports: ds_hash_file, ds_hash_directory_batch, ds_hash_directory_combined
- BLAKE3 hashing for files and directory trees
- Hidden file filtering (respects .hidden and _prefix files)
- Prometheus-compatible metrics export
- Comprehensive integration tests (12 tests)
- Benchmarks: hash_file_1kb (~14µs), hash_file_1mb (~610µs), dir_100files (~1.6ms)
## queue_index (priority queue)
- FFI exports: 25+ functions matching C++ API
- Lifecycle: qi_open, qi_close
- Task ops: add_tasks, update_tasks, remove_tasks, get_task_by_id
- Queue ops: get_next_batch, peek_next, mark_completed
- Priority: get_next_priority_task, peek_priority_task
- Query: get_all_tasks, get_tasks_by_status, get_task_count
- Retry/DLQ: retry_task, move_to_dlq
- Lease: renew_lease, release_lease
- Maintenance: rebuild_index, compact_index
- BinaryHeap-based priority queue with correct Ord (max-heap)
- Memory-mapped storage with safe Rust wrappers
- Panic-safe FFI boundaries using catch_unwind
- Comprehensive integration tests (7 tests, 1 ignored for persistence)
- Benchmarks: add_100 (~60µs), get_10 (~24ns), priority (~5µs)
## Architecture
- Cargo workspace with shared common crate
- Criterion benchmarks for both crates
- Rust 1.85.0 toolchain pinned
- Zero compiler warnings
- All 19 tests passing
Compare: make compare-benchmarks (Rust/Go/C++ comparison)
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