Fix frozen delta watermark and add error stats, lazy source, concurrent disk reads, and per-engine config

src/sqlmem/executor.py

@@ -1,4 +1,4 @@
-import sqlite3
+from typing import Any
 
 from loguru import logger
 
@@ -14,7 +14,7 @@ class QueryExecutor:
         self,
         cache: CacheManager,
         registry: ColumnRegistry,
-        source_conn: sqlite3.Connection,
+        source_conn: Any,  # raw DBAPI connection (pyodbc/sqlite3/…) — only .execute() is used
         stats: StatsCollector,
         delta: dict[str, ResolvedDelta] | None = None,
         ttl: dict[str, int] | None = None,