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_wire_insert_stress.nim

@@ -0,0 +1,26 @@
+## Regression: sequential wire-style INSERTs must not crash the process.
+## Root cause was ORC cycle collector (markGray SIGSEGV); project uses --mm:arc.
+import std/unittest
+import std/os
+import barabadb/storage/lsm
+import barabadb/query/executor
+import barabadb/query/parser
+
+proc execSql(ctx: ExecutionContext, sql: string): ExecResult =
+  executeQuery(ctx, parse(sql))
+
+suite "Wire insert stress (ARC regression)":
+  test "200 sequential INSERTs via executor survive":
+    ## Mirrors the wire path (executeQuery under StorageGate each time).
+    let dir = "/tmp/baradb_wire_stress"
+    removeDir(dir)
+    var db = newLSMTree(dir, walSyncMode = wsmNone)
+    var ctx = newExecutionContext(db)
+    check execSql(ctx, "CREATE TABLE stress (id INT PRIMARY KEY, v TEXT)").success
+    for i in 0 ..< 200:
+      let r = execSql(ctx, "INSERT INTO stress (id, v) VALUES (" & $i & ", 'v" & $i & "')")
+      check r.success
+    let sel = execSql(ctx, "SELECT id FROM stress")
+    check sel.success
+    check sel.rows.len == 200
+    db.close()