feat: harden storage, schema persistence, fair benches, fix wire crash

Core storage: hash MemTable, WAL group commit, L0 compaction rebuild,
reader-writer lock, and a global StorageGate so HTTP workers and TCP
share the LSM safely under multi-thread access.

Schema: durable CREATE/ALTER/DROP under _schema:tables:* with full LSM
restore on open. Executor types/values/schema split into query/exec/.

Wire protocol: switch default MM to ARC — ORC cycle collector segfaulted
after ~20 async INSERTs. Fair multi-tier benchmarks (SQLite/HTTP/wire/PG)
and honesty docs for mixed-tier comparisons.

tests/test_schema_persist.nim

@@ -0,0 +1,116 @@
+## Schema persistence — CREATE TABLE / data survive reopen
+import std/unittest
+import std/os
+import std/strutils
+import std/tables
+import barabadb/storage/lsm
+import barabadb/query/executor
+import barabadb/query/parser
+import barabadb/query/ast
+
+proc execSql(ctx: ExecutionContext, sql: string): ExecResult =
+  let node = parse(sql)
+  result = executeQuery(ctx, node)
+
+suite "Schema persistence":
+  test "CREATE TABLE survives flush + reopen":
+    let dir = "/tmp/baradb_schema_persist_1"
+    removeDir(dir)
+    block:
+      var db = newLSMTree(dir, 1024)  # small memtable → forces flush
+      var ctx = newExecutionContext(db)
+      let r = execSql(ctx, "CREATE TABLE users (id INT PRIMARY KEY, name TEXT NOT NULL)")
+      check r.success
+      check ctx.tables.hasKey("users")
+      check ctx.tables["users"].columns.len == 2
+      discard execSql(ctx, "INSERT INTO users (id, name) VALUES (1, 'Alice')")
+      discard execSql(ctx, "INSERT INTO users (id, name) VALUES (2, 'Bob')")
+      db.flush()
+      # Schema key must be durable
+      let (found, _) = db.get(tableSchemaKey("users"))
+      check found
+      db.close()
+
+    # Reopen fresh context (simulates process restart)
+    block:
+      var db2 = newLSMTree(dir, 1024)
+      var ctx2 = newExecutionContext(db2)
+      check ctx2.tables.hasKey("users")
+      check ctx2.tables["users"].columns.len == 2
+      check ctx2.tables["users"].pkColumns.len == 1
+      let sel = execSql(ctx2, "SELECT id, name FROM users ORDER BY id")
+      check sel.success
+      check sel.rows.len == 2
+      db2.close()
+
+  test "DROP TABLE removes schema and data":
+    let dir = "/tmp/baradb_schema_persist_drop"
+    removeDir(dir)
+    var db = newLSMTree(dir)
+    var ctx = newExecutionContext(db)
+    check execSql(ctx, "CREATE TABLE t (id INT PRIMARY KEY)").success
+    check execSql(ctx, "INSERT INTO t (id) VALUES (1)").success
+    check execSql(ctx, "DROP TABLE t").success
+    check not ctx.tables.hasKey("t")
+    let (found, _) = db.get(tableSchemaKey("t"))
+    check not found
+    # Reopen — table must not reappear
+    db.close()
+    var db2 = newLSMTree(dir)
+    var ctx2 = newExecutionContext(db2)
+    check not ctx2.tables.hasKey("t")
+    db2.close()
+
+  test "ALTER TABLE ADD COLUMN is persisted":
+    let dir = "/tmp/baradb_schema_persist_alter"
+    removeDir(dir)
+    block:
+      var db = newLSMTree(dir)
+      var ctx = newExecutionContext(db)
+      check execSql(ctx, "CREATE TABLE items (id INT PRIMARY KEY)").success
+      check execSql(ctx, "ALTER TABLE items ADD COLUMN label TEXT").success
+      check ctx.tables["items"].columns.len == 2
+      db.flush()
+      db.close()
+    block:
+      var db2 = newLSMTree(dir)
+      var ctx2 = newExecutionContext(db2)
+      check ctx2.tables.hasKey("items")
+      check ctx2.tables["items"].columns.len == 2
+      var names: seq[string] = @[]
+      for c in ctx2.tables["items"].columns:
+        names.add(c.name)
+      check "label" in names
+      db2.close()
+
+  test "Multiple tables all restored":
+    let dir = "/tmp/baradb_schema_persist_multi"
+    removeDir(dir)
+    block:
+      var db = newLSMTree(dir, 512)
+      var ctx = newExecutionContext(db)
+      check execSql(ctx, "CREATE TABLE a (id INT PRIMARY KEY)").success
+      check execSql(ctx, "CREATE TABLE b (id INT PRIMARY KEY, a_id INT)").success
+      check execSql(ctx, "CREATE TABLE c (name TEXT)").success
+      for i in 0..20:
+        discard execSql(ctx, "INSERT INTO a (id) VALUES (" & $i & ")")
+      db.flush()
+      db.close()
+    block:
+      var db2 = newLSMTree(dir)
+      var ctx2 = newExecutionContext(db2)
+      check ctx2.tables.hasKey("a")
+      check ctx2.tables.hasKey("b")
+      check ctx2.tables.hasKey("c")
+      let sel = execSql(ctx2, "SELECT id FROM a")
+      check sel.success
+      check sel.rows.len == 21
+      db2.close()
+
+  test "Stable schema key format":
+    check tableSchemaKey("users") == "_schema:tables:users"
+    check serializeTableDdl(TableDef(
+      name: "t",
+      columns: @[ColumnDef(name: "id", colType: "INT", isPk: true)],
+      pkColumns: @["id"],
+    )).contains("PRIMARY KEY")