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/protocol/zerocopy.nim

@@ -176,7 +176,8 @@ proc encodeRecord*(buf: var ZeroBuf, schema: ZcSchema,
         var v: int64 = 0
         bigEndian64(addr buf.data[field.offset], unsafeAddr v)
     of ztString:
-      buf.data[field.offset] = byte(value.len)
+      var len32 = int32(value.len)
+      bigEndian32(addr buf.data[field.offset], unsafeAddr len32)
       if value.len > 0:
         copyMem(addr buf.data[field.offset + 4], unsafeAddr value[0], value.len)
     else: