Add midnight UTC log rotation for auto-named output files

When -o is not set, the collector detects a UTC date change at the
top of each collection cycle and opens a fresh timestamped file under
logs/.  Explicit -o paths are never rotated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

collect.py

@@ -368,6 +368,11 @@ def save_store(path: Path, store: dict, max_records: int | None) -> None:
 
 # ── main loop ────────────────────────────────────────────────────────────────
 
+def _auto_out_path(logs_dir: Path, vin: str) -> Path:
+    stamp = datetime.now(timezone.utc).strftime("%Y_%m_%d_%H_%M_%S")
+    return logs_dir / f"{stamp}_{vin}.json"
+
+
 def run(args: argparse.Namespace) -> None:
     logging.basicConfig(
         level=logging.DEBUG if args.verbose else logging.INFO,
@@ -413,12 +418,13 @@ def run(args: argparse.Namespace) -> None:
 
     if args.output:
         out = Path(args.output)
+        logs_dir = None
     else:
         logs_dir = Path("logs")
         logs_dir.mkdir(exist_ok=True)
-        stamp = datetime.now().strftime("%Y_%m_%d_%H_%M_%S")
-        out = logs_dir / f"{stamp}_{vin}.json"
+        out = _auto_out_path(logs_dir, vin)
 
+    current_day = datetime.now(timezone.utc).date()
     store = load_store(out, vin, args.interval)
     log.info(
         "Collecting [%s] every %ds → %s  (Ctrl-C to stop)",
@@ -429,6 +435,14 @@ def run(args: argparse.Namespace) -> None:
 
     try:
         while True:
+            if logs_dir is not None:
+                today = datetime.now(timezone.utc).date()
+                if today != current_day:
+                    out = _auto_out_path(logs_dir, vin)
+                    store = load_store(out, vin, args.interval)
+                    current_day = today
+                    log.info("Midnight UTC rotation → %s", out)
+
             try:
                 wc.update()
                 snapshot = collect_snapshot(vehicle, domains)