fix: avoid reading correction settings when TIS is disabled

[yuyz-cyber]

Problem

When --get-mismatch-metrics uses the example compute_mis_weights_with_cp helper with TIS disabled, training fails on the first batch:

AttributeError: 'Namespace' object has no attribute 'tis_lower_bound'

compute_mis_weights reads the correction bounds before checking args.use_tis. These settings are not defined or used when TIS is disabled, so the premature access raises an exception and prevents the mismatch metrics from being returned.

Fix

Move the correction-bound calculation after the args.use_tis check. With use_tis=False, the helper returns the mismatch metrics and original loss masks without accessing correction settings. The TIS-enabled path is unchanged.

Test plan

• Added a regression test for use_tis=False without correction settings.
• Verified that mismatch metrics and the original loss masks are returned.
• tests/test_rollout_metrics.py: 12 passed.
• Ruff, Black, and isort checks passed.