cold rebuild-fts: fix NameError (missing import time)

The parallelized _cmd_cold_rebuild_fts calls time.time() but cli.py has
no module-level `import time` (functions import it locally), so the
command crashed with NameError. The command had never actually run via
the CLI before — the full recovery used a standalone script and tests
called _rebuild_fts_on_target directly — so the gap shipped in 1547259.
Now exercised end-to-end: rebuild-fts on the 4-shard corpus runs 4-way
parallel (~294s) and `cold verify` passes.

arborist/cli.py

@@ -3156,6 +3156,7 @@ def _cmd_cold_rebuild_fts(args: argparse.Namespace) -> int:
     this overwrites it cleanly. Run after `cold unpack --hydrate-shards-dir`.
     """
     import concurrent.futures as _cf
+    import time
 
     shards_dir = Path(args.shards_dir).expanduser()
     db_files = sorted(shards_dir.glob("00[0-9].db"))