Add disk-backed SQLite cache mode as an alternative to in-memory

tests/test_engine.py

@@ -289,3 +289,45 @@ def test_invalidate_then_refetch_works(engine):
 
 def test_invalidate_unknown_table_is_noop(engine):
     engine.invalidate("nonexistent_table")  # must not raise
+
+
+# ---------------------------------------------------------------------------
+# Disk-backed cache (in_memory=False)
+# ---------------------------------------------------------------------------
+
+def test_disk_mode_query_works(source_engine, cache_path, monkeypatch):
+    monkeypatch.setattr(eng_mod, "CACHE_DB_PATH", cache_path)
+    monkeypatch.setattr(eng_mod, "BACKUP_INTERVAL_SECONDS", 9999)
+
+    ce = CachingEngine(source_engine, in_memory=False)
+    rows = ce.execute("SELECT id, name FROM products")
+    assert {r["name"] for r in rows} == {"Widget", "Gadget", "Doohickey"}
+    assert ce._cache._in_memory is False
+    ce.close()
+
+
+def test_disk_mode_persists_across_instances(source_engine, cache_path, monkeypatch):
+    monkeypatch.setattr(eng_mod, "CACHE_DB_PATH", cache_path)
+    monkeypatch.setattr(eng_mod, "BACKUP_INTERVAL_SECONDS", 9999)
+
+    ce1 = CachingEngine(source_engine, in_memory=False)
+    ce1.execute("SELECT id, name FROM products")
+    ce1.close()
+
+    # Second instance opens the same on-disk cache and finds the table already there.
+    ce2 = CachingEngine(source_engine, in_memory=False)
+    assert ce2._cache.is_table_cached("products") is True
+    rows = ce2.execute("SELECT id, name FROM products")
+    assert {r["name"] for r in rows} == {"Widget", "Gadget", "Doohickey"}
+    ce2.close()
+
+
+def test_in_memory_override_respects_config(source_engine, cache_path, monkeypatch):
+    """in_memory=None falls back to the IN_MEMORY config default."""
+    monkeypatch.setattr(eng_mod, "CACHE_DB_PATH", cache_path)
+    monkeypatch.setattr(eng_mod, "BACKUP_INTERVAL_SECONDS", 9999)
+    monkeypatch.setattr(eng_mod, "IN_MEMORY", False)
+
+    ce = CachingEngine(source_engine)  # no explicit in_memory
+    assert ce._cache._in_memory is False
+    ce.close()