feat: add incremental log scanning with resumable byte offsets

Track last processed byte offset per log file so scans only read new
entries, add --full flag to force a rescan, and persist state atomically.

app/npm.py

@@ -151,6 +151,8 @@ def main() -> None:
                         help=f"minimum error responses to flag (default: {DEFAULT_MIN_ERRORS})")
     parser.add_argument("--error-ratio", type=float, default=DEFAULT_ERROR_RATIO,
                         help=f"minimum error ratio to flag (default: {DEFAULT_ERROR_RATIO})")
+    parser.add_argument("--full", action="store_true",
+                        help="rescan all logs from the start (ignore incremental state)")
     args = parser.parse_args()
 
     NpmAnalyzer(
@@ -158,6 +160,7 @@ def main() -> None:
         report_dir=args.report_dir,
         top=args.top,
         quiet=args.quiet,
+        full=args.full,
         min_errors=args.min_errors,
         error_ratio=args.error_ratio,
     ).run()