feat: compaction, page cache, WebSocket, rate limiter, TF-IDF, fuzzy search, regex, metadata filter — 73 tests

- SSTable compaction: size-tiered strategy, level-based scheduling
- Page cache: LRU eviction, hit rate tracking, capacity management
- WebSocket: full duplex streaming, frame encoding, ping/pong
- Rate limiter: token bucket + sliding window algorithms
- FTS: TF-IDF ranking, Levenshtein fuzzy matching, wildcard regex
- Vector: metadata filtering on HNSW search
- 16 new tests (73 total, all passing)

src/barabadb/vector/engine.nim

@@ -21,6 +21,7 @@ type
   HNSWNode* = ref object
     id*: uint64
     vector*: Vector
+    metadata*: Table[string, string]
     neighbors*: seq[seq[uint64]]  # neighbors per level
 
   HNSWIndex* = ref object
@@ -102,8 +103,9 @@ proc randomLevel(maxLevel: int): int =
     r = rand(1.0)
   return level
 
-proc insert*(idx: HNSWIndex, id: uint64, vector: Vector) =
-  let node = HNSWNode(id: id, vector: vector, neighbors: @[])
+proc insert*(idx: HNSWIndex, id: uint64, vector: Vector,
+             metadata: Table[string, string] = initTable[string, string]()) =
+  let node = HNSWNode(id: id, vector: vector, metadata: metadata, neighbors: @[])
   let level = randomLevel(16)
 
   for i in 0..level:
@@ -137,6 +139,23 @@ proc search*(idx: HNSWIndex, query: Vector, k: int,
 
   return candidates
 
+proc searchWithFilter*(idx: HNSWIndex, query: Vector, k: int,
+                       filter: proc(metadata: Table[string, string]): bool {.gcsafe.},
+                       metric: DistanceMetric = dmCosine): seq[(uint64, float64)] =
+  if idx.nodes.len == 0:
+    return @[]
+
+  var candidates: seq[(uint64, float64)] = @[]
+  for nodeId, node in idx.nodes:
+    if filter(node.metadata):
+      let dist = distance(query, node.vector, metric)
+      candidates.add((nodeId, dist))
+
+  candidates.sort(proc(a, b: (uint64, float64)): int = cmp(a[1], b[1]))
+  if candidates.len > k:
+    candidates = candidates[0..<k]
+  return candidates
+
 proc newIVFPQIndex*(dimensions: int, nClusters: int = 100,
                     nSubquantizers: int = 8, nBits: int = 8,
                     metric: DistanceMetric = dmCosine): IVFPQIndex =