feat: implement snapshot-based persistence (closes #39)

[ithustle]

Summary

• Add snapshot persistence that serializes the store and expires hashtables
  to a binary RDB file with CRC32 integrity checking
• Atomic writes via tmp file + fsync + rename to prevent corruption on crash
• Graceful shutdown saves snapshot before exit (SIGINT sets flag, event loop
  calls shutdown_server)
• Periodic save triggered from the 100ms timer in each event dispatcher when
  dirty >= threshold and enough ticks have elapsed
• Snapshot loaded on startup, expired keys discarded during load
• Enabled by default: save-interval 300 1 (5 min, 1 change minimum)
• Configurable via save-interval, snapshot-path in server.conf;
  disable with save-interval 0 0

Files added (5)

• src/persistence/crc32.h — CRC32 lookup table and update function
• src/persistence/persistence.h — save/load API
• src/persistence/persistence.c — serialization, atomic write, CRC verification
• src/fkvs_time.h — standalone fkvs_now_ms() to decouple persistence from utils.h
• tests/test_persistence.c — 8 test cases (round-trip, encoding, TTL, corruption)

Files modified (8)

• src/server.h — new fields: save_interval, dirty, shutdown_requested, snapshot_path
• src/config.c — defaults and parsing for save-interval, snapshot-path
• src/server.c — shutdown_server(), load on startup, simplified SIGINT handler
• src/networking/networking.c — dirty counter for write commands
• src/io/event_dispatcher_{kqueue,epoll,io_uring}.c — shutdown check + periodic save
• src/utils.h — delegates fkvs_now_ms() to fkvs_time.h
• CMakeLists.txt — persistence.c in server targets, test_persistence target
• server.conf — documented save-interval and snapshot-path options

RDB format

HEADER: [4B magic "FKVS"] [1B version] [8B timestamp BE]
STORE: [1B marker 0x01] [8B count BE] [entries: key_len|key|val_len|val|encoding]
EXPIRES: [1B marker 0x02] [8B count BE] [entries: key_len|key|deadline_ms BE]
FOOTER: [4B CRC32 BE] [1B EOF 0xFF]

Test plan

• test_persistence — 8 unit tests covering round-trip, encoding, TTL,
  expired discard, empty DB, missing file, corrupted header, corrupted CRC
• Manual: start server with default config, SET keys, wait >5min or
  Ctrl+C, verify fkvs-dump.rdb created, restart and GET keys back
• Manual: set save-interval 0 0, verify no snapshot created

Issue reference: #39