Cut portal churn residue that leaks into typing latency

[arzafran]

What this does

The typing-lag gate on main failed first-attempt on both post-glass merges (3.57x, then 2.44x vs the 1.70x cap) and passed on rerun — the glass portal raised workspace-churn typing latency into gate-adjacent territory. This removes the two cheapest real costs the portal adds during churn. Both changes are strictly less work; no behavior changes.

Summary

• Reassert requests (posted per pane close) are now scoped to the closing window instead of broadcast app-wide, so churn no longer triggers republish storms across every workspace. The dismantle-race survivor the mechanism protects is always in the same window; a nil object still broadcasts.
• Control clusters no longer re-add themselves to the host view on every coalesced geometry pass (re-adding an existing subview removes and re-inserts it, dirtying layout per runloop turn during animation storms); they reorder only when a newly added pane bar actually sits above them.

Test plan

• CI green, including tests-build-and-lag on the merge run
• Manual: split churn (split, close, repeat) — pills and control capsules stay positioned and clickable; closing a pane during split churn still restores the surviving pane's tab bar (the reassert race)