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/storage/btree.nim

@@ -62,9 +62,12 @@ proc splitChild[K, V](parent: BTreeNode[K, V], index: int, order: int) =
   let midKey = child.keys[mid]
   parent.keys.insert(midKey, index)
   parent.children.insert(newNode, index + 1)
-  child.keys.setLen(mid)
+  # In B+ tree, leaf nodes must keep the boundary key for range scans
   if child.isLeaf:
-    child.values.setLen(mid)
+    child.keys.setLen(mid + 1)
+    child.values.setLen(mid + 1)
+  else:
+    child.keys.setLen(mid)
 
 proc insertNonFull[K, V](node: BTreeNode[K, V], key: K, value: V, order: int) =
   var i = node.keys.len - 1