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

tests/test_stats.py

@@ -73,6 +73,29 @@ def test_counters_still_reported(source_engine, patched_cache):
     engine.close()
 
 
+def test_stats_exposes_table_error(source_engine, patched_cache):
+    engine = CachingEngine(source_engine)
+    engine.execute("SELECT id, name FROM products")
+    engine._cache.record_error("products", "ValueError: boom")
+
+    s = engine.stats
+    assert s.errors == 1
+    assert s.tables["products"].consecutive_failures == 1
+    assert s.tables["products"].last_error == "ValueError: boom"
+    assert s.tables["products"].last_error_at is not None
+    engine.close()
+
+
+def test_stats_no_error_by_default(source_engine, patched_cache):
+    engine = CachingEngine(source_engine)
+    engine.execute("SELECT id, name FROM products")
+    s = engine.stats
+    assert s.errors == 0
+    assert s.tables["products"].consecutive_failures == 0
+    assert s.tables["products"].last_error is None
+    engine.close()
+
+
 # --- a table being loaded for the first time shows up as "loading" ----------