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

tests/test_engine.py

@@ -124,6 +124,22 @@ def test_second_query_same_columns_is_cache_hit(engine):
     assert len(rows) == 3
 
 
+def test_cache_hit_does_not_open_source(engine, source_engine, monkeypatch):
+    """A pure cache hit must not open a source connection (lazy source)."""
+    engine.execute("SELECT id, name FROM products")  # miss → caches
+
+    calls = {"n": 0}
+    original_connect = source_engine.connect
+
+    def counting_connect(*args, **kwargs):
+        calls["n"] += 1
+        return original_connect(*args, **kwargs)
+
+    monkeypatch.setattr(source_engine, "connect", counting_connect)
+    engine.execute("SELECT id, name FROM products")  # hit → no source access
+    assert calls["n"] == 0
+
+
 # ---------------------------------------------------------------------------
 # SQL file creation — backup to disk
 # ---------------------------------------------------------------------------
@@ -331,3 +347,41 @@ def test_in_memory_override_respects_config(source_engine, cache_path, monkeypat
     ce = CachingEngine(source_engine)  # no explicit in_memory
     assert ce._cache._in_memory is False
     ce.close()
+
+
+# ---------------------------------------------------------------------------
+# Per-engine configuration (constructor overrides env defaults)
+# ---------------------------------------------------------------------------
+
+def test_constructor_config_overrides(source_engine, tmp_path):
+    p = tmp_path / "explicit_cache.db"
+    ce = CachingEngine(
+        source_engine,
+        cache_db_path=p,
+        fetch_batch=3,
+        dialect="sqlite",
+        backup_interval=12345,
+        refresh_interval=42,
+        in_memory=False,
+    )
+    ce.execute("SELECT id, name FROM products")
+    assert p.exists()
+    assert ce._cache._fetch_batch == 3
+    assert ce._cache._dialect == "sqlite"
+    assert ce._dialect == "sqlite"
+    assert ce._cache._backup_interval == 12345
+    assert ce._refresh_interval == 42
+    ce.close()
+
+
+def test_two_engines_separate_cache_files(source_engine, tmp_path):
+    """Two engines in one process can target different cache files."""
+    a = CachingEngine(source_engine, cache_db_path=tmp_path / "a.db", in_memory=False)
+    b = CachingEngine(source_engine, cache_db_path=tmp_path / "b.db", in_memory=False)
+    a.execute("SELECT id FROM products")
+
+    assert (tmp_path / "a.db").exists()
+    assert a._cache.is_table_cached("products") is True
+    assert b._cache.is_table_cached("products") is False  # independent cache
+    a.close()
+    b.close()