Track pause ownership so outro walks can't undo each other

Describe the problem

Pause and resume walks in the transition machinery don't know who paused a subtree, so each controller keeps its own guard against undoing someone else's state, branches.js tracks an #outroing set and each.js checks INERT at two call sites. The fixes in this area (#16150, #17258, #17942, #18291) are mostly walks stepping on each other's state one case at a time, and #15604 is the same class, resume_children resurrecting a subtree that outroed for its own reasons.

Describe the proposed solution

#18431 adds a PAUSED bit so resume_children stops at roots that paused independently. Making pause_effect a no-op on an already paused root would finish the job, the per-controller guards could then collapse onto the flag instead of each tracking pause state separately. Happy to PR it on top of #18431 if that's a direction you want.

Alternatives considered

Keeping the per-controller guards. They work, but each new controller has to rediscover the problem, which is how the point fix history accumulated.

Importance

nice to have