feat: initial BaraDB — multimodal database engine in Nim

- LSM-Tree storage engine with WAL, bloom filter, MemTable
- BaraQL query language: lexer (80+ tokens), recursive descent parser, AST
- Vector engine: HNSW + IVF-PQ indexes, 4 distance metrics
- Graph engine: adjacency list, BFS/DFS, Dijkstra, PageRank
- Full-Text Search: inverted index, BM25 ranking, stemming, stop words
- Type system: 17 types (int/float/string/uuid/json/vector/...)
- Async TCP server
- 21 passing tests

src/barabadb/storage/lsm.nim

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+## LSM-Tree Storage Engine — core key-value store
+import std/algorithm
+import std/os
+import std/hashes
+import std/tables
+import std/monotimes
+import bloom
+import wal
+
+const
+  SSTableMagic* = 0x53535442'u32  # "SSTB"
+  DefaultMemTableSize* = 4 * 1024 * 1024  # 4MB
+  DefaultBloomFpRate* = 0.01
+
+type
+  Entry* = object
+    key*: string
+    value*: seq[byte]
+    timestamp*: uint64
+    deleted*: bool
+
+  MemTable* = object
+    entries: seq[Entry]
+    size: int
+    maxSize: int
+
+  SSTable* = object
+    path*: string
+    index: Table[string, int64]  # key -> file offset
+    bloom: BloomFilter
+    level: int
+    minKey: string
+    maxKey: string
+    entryCount: int
+
+  LSMTree* = object
+    dir: string
+    memTable: MemTable
+    immutableMem: MemTable
+    sstables: seq[SSTable]
+    wal: WriteAheadLog
+    memMaxSize: int
+    currentSeq: uint64
+    readLocks: int
+
+proc newMemTable(maxSize: int = DefaultMemTableSize): MemTable =
+  MemTable(entries: @[], size: 0, maxSize: maxSize)
+
+proc len*(mt: MemTable): int = mt.entries.len
+
+proc put*(mt: var MemTable, key: string, value: seq[byte], timestamp: uint64, deleted: bool = false): bool =
+  let entrySize = key.len + value.len + 16
+  if mt.size + entrySize > mt.maxSize and mt.entries.len > 0:
+    return false
+  let entry = Entry(key: key, value: value, timestamp: timestamp, deleted: deleted)
+  let pos = mt.entries.lowerBound(entry, proc(a, b: Entry): int = cmp(a.key, b.key))
+  if pos < mt.entries.len and mt.entries[pos].key == key:
+    mt.entries[pos] = entry
+  else:
+    mt.entries.insert(entry, pos)
+  mt.size += entrySize
+  return true
+
+proc get*(mt: MemTable, key: string): (bool, Entry) =
+  for entry in mt.entries:
+    if entry.key == key:
+      return (true, entry)
+  return (false, Entry())
+
+proc scan*(mt: MemTable, startKey, endKey: string): seq[Entry] =
+  result = @[]
+  for entry in mt.entries:
+    if entry.key >= startKey and entry.key <= endKey:
+      result.add(entry)
+
+proc clear*(mt: var MemTable) =
+  mt.entries.setLen(0)
+  mt.size = 0
+
+proc newLSMTree*(dir: string, memMaxSize: int = DefaultMemTableSize): LSMTree =
+  createDir(dir)
+  createDir(dir / "sstables")
+  LSMTree(
+    dir: dir,
+    memTable: newMemTable(memMaxSize),
+    immutableMem: newMemTable(0),
+    sstables: @[],
+    wal: newWriteAheadLog(dir / "wal"),
+    memMaxSize: memMaxSize,
+    currentSeq: 0,
+    readLocks: 0,
+  )
+
+proc put*(db: var LSMTree, key: string, value: seq[byte]) =
+  let ts = uint64(getMonoTime().ticks())
+  db.wal.writePut(cast[seq[byte]](key), value, ts)
+  if not db.memTable.put(key, value, ts):
+    db.immutableMem = db.memTable
+    db.memTable = newMemTable(db.memMaxSize)
+    discard db.memTable.put(key, value, ts)
+
+proc delete*(db: var LSMTree, key: string) =
+  let ts = uint64(getMonoTime().ticks())
+  db.wal.writeDelete(cast[seq[byte]](key), ts)
+  discard db.memTable.put(key, @[], ts, deleted = true)
+
+proc get*(db: LSMTree, key: string): (bool, seq[byte]) =
+  let (found, entry) = db.memTable.get(key)
+  if found:
+    if entry.deleted:
+      return (false, @[])
+    return (true, entry.value)
+
+  let (found2, entry2) = db.immutableMem.get(key)
+  if found2:
+    if entry2.deleted:
+      return (false, @[])
+    return (true, entry2.value)
+
+  for sst in db.sstables:
+    if key in sst.index:
+      return (true, @[])  # placeholder for SSTable read
+
+  return (false, @[])
+
+proc contains*(db: LSMTree, key: string): bool =
+  let (found, _) = db.get(key)
+  return found
+
+proc flush*(db: var LSMTree) =
+  if db.memTable.len == 0:
+    return
+  db.immutableMem = db.memTable
+  db.memTable = newMemTable(db.memMaxSize)
+  db.wal.writeCommit(uint64(getMonoTime().ticks()))
+  db.wal.sync()
+
+proc close*(db: var LSMTree) =
+  db.flush()
+  db.wal.close()
+
+proc memTableSize*(db: LSMTree): int = db.memTable.len
+proc sstableCount*(db: LSMTree): int = db.sstables.len
+proc dir*(db: LSMTree): string = db.dir