perf: bound lake scans and add platform Grafana dashboard

Explorer and exporter were scanning the full Parquet lake every 1–5s
(~2000 files, 200MB+), driving ~2.2 CPU cores. Limit metrics to the
last N flights, cache DuckDB views and the partition tree, slow live
polls to 2s, and keep drones alive after seal to stop restart churn.

Add node-exporter, scan-duration metrics, and a Swarm Platform Grafana
dashboard for node CPU/memory and scan health.

simulator/tests/test_lake.py

@@ -0,0 +1,40 @@
+"""Lake scan helpers — bounded to recent flight partitions."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from monitoring.lake import flight_window, iter_parquet_files, parquet_reader, recent_flights
+
+
+def test_parquet_reader_limits_to_recent_flights(tmp_path: Path) -> None:
+    for i, name in enumerate(("flight=aaa", "flight=bbb", "flight=ccc")):
+        hour = tmp_path / f"dataset=telemetry/{name}/drone=dr-01/sensor=imu/year=2026/month=07/day=08/hour=10"
+        hour.mkdir(parents=True)
+        f = hour / "data.parquet"
+        f.write_bytes(b"x" * (i + 1))
+        # Make later names newer
+        import os
+        import time
+        os.utime(f, (time.time() + i, time.time() + i))
+
+    reader = parquet_reader(tmp_path, "telemetry", limit=1)
+    assert "flight=ccc" in reader
+    assert "flight=bbb" not in reader
+
+
+def test_iter_parquet_files_skips_old_flights(tmp_path: Path) -> None:
+    old = tmp_path / "dataset=state/flight=old/drone=dr-01/year=2026/month=07/day=08/hour=09"
+    old.mkdir(parents=True)
+    (old / "data.parquet").write_bytes(b"old")
+    new = tmp_path / "dataset=state/flight=new/drone=dr-01/year=2026/month=07/day=08/hour=10"
+    new.mkdir(parents=True)
+    new_file = new / "data.parquet"
+    new_file.write_bytes(b"new")
+    import os
+    import time
+    os.utime(new_file, (time.time() + 10, time.time() + 10))
+
+    files = iter_parquet_files(tmp_path, flight_limit=1)
+    assert len(files) == 1
+    assert "flight=new" in str(files[0])