feat(sol-initiated): per-category self-mute clear markers
Rename category_self_mute_clear_marker_ts (scalar) to
category_self_mute_clear_markers (dict[str, int]). The lode-3 settings
UI needs to clear self-mute per category, not globally. No back-compat
shim — lode 1 just shipped and there are no users to migrate. Adds
compute_category_mute_state() helper so policy and the new settings API
share one source of truth for mute state.