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/core/raft.nim

@@ -64,6 +64,7 @@ type
     messageQueue*: Deque[RaftMessage]
 
 proc newRaftNode*(id: string, peers: seq[string]): RaftNode =
+  randomize()
   RaftNode(
     id: id,
     state: rsFollower,
@@ -211,8 +212,9 @@ proc appendEntries*(node: RaftNode, peerId: string): RaftMessage =
     prevTerm = node.log[prevIdx - 1].term
 
   var entries: seq[LogEntry] = @[]
-  for i in int(nextIdx - 1)..<node.log.len:
-    entries.add(node.log[i])
+  if nextIdx > 0:
+    for i in int(nextIdx - 1)..<node.log.len:
+      entries.add(node.log[i])
 
   return RaftMessage(
     kind: rmkAppendEntries,