Стабилизация на storage слоя — 6 фази

Фаза 1: SSTable Integrity
- SSTable v3 формат с CRC32 footer (data/index/bloom CRC)
- Нов модул storage/crc32.nim (zero-dep, IEEE 802.3)
- verifySSTable() — проверка на magic, version, CRC
- loadSSTable strict mode — отхвърля корумпирани файлове
- newLSMTree логва [WARN] при corrupt SSTables

Фаза 2: baradb repair
- Нов модул tools/repair.nim
- Сканира SSTables, проверява CRC, премества битите в corrupt/
- WAL replay (CrashRecovery) за възстановяване на данни
- CLI: ./baradadb repair --data-dir=... [--dry-run]

Фаза 3: MANIFEST File
- Atomic write MANIFEST (tmp + rename)
- newLSMTree зарежда от MANIFEST, fallback към walkDir
- checkStorageConsistency() — orphan/missing detection
- flushUnsafe и compaction записват MANIFEST

Фаза 4: WAL Rotation & Incremental Backup
- WAL segment rotation при 64MB (wal_archive/wal.000NNN.log)
- maybeRotate() на всеки 1000 записа + при flush
- backup incremental — архивира MANIFEST + SSTables + WAL
- CRC verification на SSTables преди архивиране

Фаза 5: Online Consistent Backup
- checkpoint() — freeze memtable + flush + WAL rotate
- ./baradadb checkpoint — offline consistent snapshot
- backup --online — checkpoint + incremental backup

Фаза 6: SSTable Version Migration
- SSTable.fileVersion поле
- listLegacySSTables() — намира v1/v2 файлове
- migrateSSTable() — пренаписва към v3 (tmp + rename)
- ./baradadb migrate [--dry-run] — offline migration

Документация:
- Обновени docs/en/backup.md, docs/bg/backup.md
- Обновени docs/en/storage.md, docs/bg/storage.md
- Добавени тестове в tests/test_all.nim

docs/en/storage.md

@@ -21,11 +21,105 @@ db.close()
 
 - **MemTable**: In-memory sorted buffer
 - **WAL**: Write-ahead log for durability
-- **SSTable**: Sorted string tables on disk
+- **SSTable**: Sorted string tables on disk (v3 with CRC footer)
 - **Bloom Filter**: Probabilistic set membership
 - **Compaction**: Size-tiered strategy with level management
+- **MANIFEST**: Atomic catalog of active SSTables
 - **Page Cache**: LRU cache with hit rate tracking
 
+### SSTable Format (v3)
+
+```
+[Header] 36 bytes
+  magic:       uint32  (0x53535442 = "SSTB")
+  version:     uint32  (3 = current)
+  entryCount:  uint32
+  level:       uint32
+  indexOffset: uint64
+  bloomOffset: uint64
+  footerOffset: uint64
+
+[Data Block]
+  keyLen: uint32
+  key:    bytes[keyLen]
+  valueLen: uint32
+  value:  bytes[valueLen]
+  timestamp: uint64
+  deleted: uint8
+
+[Index Block]
+  keyLen: uint32
+  key:    bytes[keyLen]
+  dataOffset: uint64
+
+[Bloom Filter Block]
+  bloomSize: uint32
+  bloomData: bytes[bloomSize]
+
+[Footer] 16 bytes
+  dataCrc32:  uint32  (CRC32 of Data Block)
+  indexCrc32: uint32  (CRC32 of Index Block)
+  bloomCrc32: uint32  (CRC32 of Bloom Block)
+  reserved:   uint32  (must be 0)
+```
+
+The CRC footer enables independent verification of each SSTable file via `verifySSTable(path)`.
+
+### MANIFEST Catalog
+
+The `MANIFEST` file tracks all active SSTables atomically:
+
+```json
+{
+  "version": 1,
+  "sequence": 42,
+  "createdAt": 1779103266,
+  "sstables": [
+    {"id": 1, "path": "sstables/1.sst", "level": 0, "minKey": "a", "maxKey": "z", "entryCount": 100}
+  ]
+}
+```
+
+- Written atomically via `MANIFEST.tmp` + rename
+- Read at startup for fast loading
+- Updated after every flush and compaction
+
+### WAL Rotation
+
+The Write-Ahead Log rotates when it reaches 64MB:
+
+```
+wal/
+  ├── wal.log          (active segment)
+  └── wal_archive/
+      ├── wal.000001.log
+      └── wal.000002.log
+```
+
+Rotation triggers:
+- Every 1000 entries (lightweight check)
+- On every `flush()` or `checkpoint()`
+
+### Storage Repair
+
+Verify and repair storage integrity:
+
+```bash
+# Check all SSTables
+./build/baradadb repair --data-dir=./data --dry-run
+
+# Full repair
+./build/baradadb repair --data-dir=./data
+```
+
+### SSTable Migration
+
+Rewrite legacy v1/v2 SSTables to v3:
+
+```bash
+./build/baradadb migrate --data-dir=./data
+```
+
 ## B-Tree Index
 
 Ordered index for range scans and point lookups.
@@ -43,14 +137,14 @@ let range = btree.scan("key_a", "key_z")
 
 ## Write-Ahead Log (WAL)
 
-Ensures durability of write operations.
+Ensures durability of write operations with segment rotation.
 
 ```nim
 import barabadb/storage/wal
 
-var wal = newWAL("./wal")
-wal.append("txn1", "SET key1 value1")
-wal.flush()
+var wal = newWriteAheadLog("./wal")
+wal.writePut(key, value, timestamp)
+wal.sync()
 ```
 
 ## Bloom Filter
@@ -73,6 +167,6 @@ Efficient file access using mmap.
 ```nim
 import barabadb/storage/mmap
 
-var mapped = mmapFile("./data/file.dat")
-let data = mapped.read(0, 100)
-```
+var mapped = openMmap("./data/file.dat")
+let val = mapped.readUint32(0)
+```