Bug fixes: composite PK, nl_to_sql sandbox, FK check, SQL injection, storage correctness

Critical fixes:
- Composite PK: execInsert + validateConstraints use all PK columns
- nl_to_sql: non-SELECT SQL no longer executed directly during validation
- FK check: removed O(N) scanMemTable fallback, uses db.get() with SSTables
- exprToSql: nkIdent wrapped in quotes to prevent SQL injection
- restoreSchema: try/except around tokenize/parse for crash resilience
- recovery.nim: lastTxnId tracking + putUnsafe/deleteUnsafe without WAL
- SCRAM: verifyClientProof length check + DefaultIterationCount restored

Storage fixes:
- lsm.nim: SSTable sort order fixed (ascending), close() flushes all memtables
- compaction.nim: tombstones preserved during compaction
- wal.nim: header written for empty existing files, readEntries checks magic
- btree.nim: B+ tree leaf split keeps boundary key
- bloom.nim: deserialize raises on short data
- mmap.nim: bounds checks for adviseWillNeed/DontNeed + posix.close()

Protocol fixes:
- zerocopy.nim: readString bounds check
- wire.nim: deserializeValue 32-bit underflow check
- auth.nim: JWT JSON escaping for claims
- server.nim: readUint32BE bounds check + specific exception handling
- raft.nim: readData checks + specific exception handling

Tests:
- Added Composite Primary Key test suite (4 tests)

Build: 0 warnings, 0 errors

src/barabadb/protocol/auth.nim

@@ -110,10 +110,13 @@ proc constantTimeCompare(a, b: string): bool =
 # JWT token helpers
 # ---------------------------------------------------------------------------
 
+proc jsonEscape(s: string): string =
+  result = s.replace("\\", "\\\\").replace("\"", "\\\"")
+
 proc createToken*(am: AuthManager, claims: JWTClaims): string =
   let header = base64UrlEncode("{\"alg\":\"HS256\",\"typ\":\"JWT\"}")
-  var payloadJson = "{\"sub\":\"" & claims.sub & "\",\"role\":\"" & claims.role &
-    "\",\"database\":\"" & claims.database & "\""
+  var payloadJson = "{\"sub\":\"" & jsonEscape(claims.sub) & "\",\"role\":\"" & jsonEscape(claims.role) &
+    "\",\"database\":\"" & jsonEscape(claims.database) & "\""
   if claims.exp > 0:
     payloadJson &= ",\"exp\":" & $claims.exp
   if claims.iat > 0: