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

src/sqlmem/_sql.py

@@ -0,0 +1,27 @@
+"""SQL identifier quoting.
+
+Table and column names are interpolated into statements as raw strings, so a
+name with a space, a reserved word, or an embedded quote would break the query
+(and is a latent injection vector). These helpers quote identifiers safely. The
+in-memory cache is SQLite, so it uses double-quote style; the source DB is quoted
+in its configured dialect (e.g. T-SQL ``[brackets]``).
+"""
+
+from collections.abc import Iterable
+
+from sqlglot import exp
+
+
+def quote(name: str) -> str:
+    """Quote an identifier for the in-memory SQLite cache."""
+    return '"' + name.replace('"', '""') + '"'
+
+
+def quote_list(names: Iterable[str]) -> str:
+    """Comma-join SQLite-quoted identifiers."""
+    return ", ".join(quote(n) for n in names)
+
+
+def quote_source(name: str, dialect: str) -> str:
+    """Quote an identifier for the source DB in its dialect (e.g. T-SQL ``[x]``)."""
+    return exp.to_identifier(name, quoted=True).sql(dialect=dialect)