perf(core): RwLock engine, fuzz testing, and more

[ElioNeto]

Summary

Three significant changes to the ApexStore storage engine:

1. STARVATION-001 — RwLock Engine Core (#360)

Changed Engine's core lock from parking_lot::Mutex to parking_lot::RwLock. This allows concurrent reads (get, scan, keys, stats, snapshots) to proceed in parallel without blocking each other. Writes (put, delete, flush, compact, batch) still acquire exclusive access via write().

• Added lock_core_mut() for write access alongside lock_core() (reads)
• Classified all 24 lock sites as read or write operations
• Updated Transaction, scrubber, tests accordingly
• All 554 unit tests pass (including previously-failing delete_range and TTL tests)

2. TEST-001 — Fuzz Testing (#371)

Added cargo-fuzz infrastructure with two fuzz targets:

• wal — fuzzes WAL write_record, write_batch, recovery, TTL records (1000 runs, 0 crashes)
• sstable — fuzzes SSTable build, get, scan, bloom filter (1000 runs, 0 crashes)

3. Remaining issues

The following issues are still open and have been deferred due to complexity:

• WRITE-WAL-001: WAL sem group commit — fsync a cada 4 writes limita throughput #362 WRITE-SERIAL-001 (Serial WAL pipeline)
• WRITE-SERIAL-001: Pipeline de escrita WAL + MemTable serializada — throughput limitado #363 WRITE-WAL-001 (Group commit)
• CRASH-SST-001: SSTable truncado é detectado mas sem recuperação automática — perda de dados #357 CRASH-SST-001 (SSTable auto-repair)
• MEM-MEMTABLE-002: MemTable usa BTreeMap O(log n) em vez de skip list lock-free — concorrência limitada #367 MEM-MEMTABLE-002 (Lock-free skip list)
• TEST-003: Adicionar chaos testing com injeção de falhas de I/O (fail-rs) #373 TEST-003 (Chaos testing)

Closes #360
Closes #371