fix: comprehensive bug audit — 15 critical/high/medium fixes

Critical fixes:
- B-Tree splitChild: fixed out-of-bounds on child.children.len (mid+1..len → mid+1..<len)
- Wire protocol: all deserializeValue cases now properly assign to result
- Wire protocol: fkBool/fkInt8 now increment pos after reading
- MVCC: write-write conflict now detected for completed transactions
- MVCC: commit only marks visible versions as deleted (not concurrent uncommitted)
- Lexer: buffer overrun on backslash escape at end of input — added bounds check
- Bulgarian/Russian stemmers: fixed UTF-8 byte offsets for Cyrillic suffixes (2 bytes per char)

High fixes:
- WebSocket: added missing std/tables import
- Raft: added randomize() call for election timeouts
- MemTable.get: binary search O(log n) instead of linear scan O(n)

Medium fixes:
- Zerocopy: string length now written as 4-byte int32 (was single byte)
- Raft appendEntries: guard against nextIdx underflow
- Columnar: null tracking with nulls seq on ColumnPtr
- Removed dead code in lexer readIdent (redundant true/false checks)
- Updated Bulgarian stemming test to match corrected byte offsets

All 214 tests pass across 48 suites with 0 failures.

src/barabadb/storage/btree.nim

@@ -48,7 +48,7 @@ proc splitChild[K, V](parent: BTreeNode[K, V], index: int, order: int) =
       newNode.values.add(child.values[j])
 
   if not child.isLeaf:
-    for j in mid+1..child.children.len:
+    for j in mid+1..<child.children.len:
       newNode.children.add(child.children[j])
     child.children.setLen(mid + 1)
 

src/barabadb/storage/lsm.nim

@@ -62,9 +62,20 @@ proc put*(mt: var MemTable, key: string, value: seq[byte], timestamp: uint64, de
   return true
 
 proc get*(mt: MemTable, key: string): (bool, Entry) =
-  for entry in mt.entries:
-    if entry.key == key:
-      return (true, entry)
+  if mt.entries.len == 0:
+    return (false, Entry())
+  # Binary search since entries are kept sorted by key
+  var lo = 0
+  var hi = mt.entries.len - 1
+  while lo <= hi:
+    let mid = (lo + hi) div 2
+    let cmp = cmp(mt.entries[mid].key, key)
+    if cmp == 0:
+      return (true, mt.entries[mid])
+    elif cmp < 0:
+      lo = mid + 1
+    else:
+      hi = mid - 1
   return (false, Entry())
 
 proc scan*(mt: MemTable, startKey, endKey: string): seq[Entry] =