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

src/sqlmem/stats.py

@@ -20,6 +20,11 @@ class TableStats:
     last_refresh: str
     state: str = TableState.READY
     tracking: str = "static"  # "delta" | "ttl" | "static"
+    # Most recent load/refresh failure for this table, if any. ``consecutive_failures``
+    # resets to 0 on the next success, so > 0 means the table is currently failing.
+    last_error: str | None = None
+    last_error_at: str | None = None
+    consecutive_failures: int = 0
 
 
 @dataclass(frozen=True)
@@ -28,6 +33,7 @@ class Stats:
     misses: int
     refetches: int
     tables: dict[str, TableStats]
+    errors: int = 0  # total load/refresh failures since start
 
 
 class StatsCollector: