feat(formatter): resumable chunk merge with bounded fan-in

Collapse chunk_*.jsonl in rounds with a bounded merge fan-in so the number of
simultaneously open chunk iterators stays flat regardless of how many chunks a
trace produces, and write a resume manifest so --resume-tmp-dir can continue
from an existing formatter temp directory without rescanning the raw trace.

trace_formatter/cli.py

@@ -64,6 +64,11 @@ def build_parser() -> argparse.ArgumentParser:
         help="Base directory used when --output is omitted. Defaults to a sibling trace-*-formatted directory.",
     )
     format_parser.add_argument("--tmp-dir", default=None)
+    format_parser.add_argument(
+        "--resume-tmp-dir",
+        default=None,
+        help="Resume from an existing formatter temp directory containing chunk_*.jsonl and skip raw trace scanning.",
+    )
     format_parser.add_argument("--chunk-bytes", type=int, default=128 * 1024 * 1024)
     format_parser.add_argument(
         "--log-file",
@@ -146,6 +151,7 @@ def main(argv: list[str] | None = None) -> int:
             input_dir=args.input,
             output_path=raw_output_path,
             tmp_dir=args.tmp_dir,
+            resume_tmp_dir=args.resume_tmp_dir,
             chunk_bytes=args.chunk_bytes,
             start_time=args.start_time,
             end_time=args.end_time,