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/mvcc.nim

@@ -170,9 +170,12 @@ proc write*(tm: TxnManager, txn: Transaction, key: string, value: seq[byte]): bo
                 if tm.activeTxns[creator].state == tsCommitted:
                   release(tm.lock)
                   return false  # write-write conflict
-              elif creator notin tm.activeTxns:
-                # Already completed and visible
-                discard
+              else:
+                # Creator already completed — check if it committed
+                # If not in activeTxns, it either committed or aborted
+                # We treat completed-but-visible as a conflict
+                release(tm.lock)
+                return false  # write-write conflict with completed txn
 
   let lsn = tm.allocLsn()
   let version = VersionedRecord(
@@ -201,9 +204,15 @@ proc commit*(tm: TxnManager, txn: Transaction): bool =
       tm.globalVersions[key] = @[]
 
     # Mark previous versions as deleted by this txn
+    # Only mark versions that were visible to this transaction
     for i in 0..<tm.globalVersions[key].len:
       if tm.globalVersions[key][i].xmax == TxnId(0):
-        tm.globalVersions[key][i].xmax = txn.id
+        let creator = tm.globalVersions[key][i].xmin
+        # Only mark as deleted if the version was visible (committed before our snapshot)
+        if creator != txn.id:
+          if uint64(creator) <= uint64(txn.snapshotMaxTxn) and
+             creator notin txn.snapshotTxns:
+            tm.globalVersions[key][i].xmax = txn.id
 
     tm.globalVersions[key].add(version)