The marquee registry stops being observable — reflow writes cost a dictionary store and nothing else

Drops @Observable from MarqueeTargetRegistry (MarqueeSession keeps it —
its rect genuinely renders the band). The audit found no body read
anywhere: the begin guard and sample loop read from inside the drag
gesture, the arrows from inside a key handler, so nothing ever needed
invalidating when a frame moved — while every make-room reflow had each
sliding face re-firing onGeometryChange per display frame, each write
paying Observation registrar bookkeeping on top of the reflow's own
render work (the confirmed A/B culprit of 2026-07-31). Write-gating on
drag-active was rejected: a suppressed write never replays, leaving the
band and arrows navigating stale rectangles. A tripwire test pins the
registry against anyone re-adding the macro.

Drag-perf confirmed culprit, card eb7b75ce.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/UI/Board/MarqueeSession.swift

@@ -93,8 +93,26 @@ final class MarqueeSession {
 /// It is also the **begin guard's** universe: a drag that starts inside a registered frame belongs
 /// to that item's own gesture (a card drag, a drag out of the trash), never to the band. Deciding
 /// that geometrically rather than by gesture priority is what keeps the two from fighting.
+///
+/// **Deliberately not `@Observable`** — `LaneDropRegistry`'s rule, for the same reason and with the
+/// same proof: nothing renders off it. Every reader asks at *event* time — the band's begin guard and
+/// its sample loop from inside a drag gesture (`MarqueeControl.gesture(in:)`), the arrows from inside
+/// a key handler (`BoardView.step`, `.extend`, over `NavigationMath`) — so no view body ever reads a
+/// frame from here and none needs invalidating when one moves.
+///
+/// What observing it cost is the whole reason the rule is stated: every card face registers here
+/// through `onGeometryChange`, and a drag's make-room reflow *animates positions*, so for the ~0.18 s
+/// of every proposal change each sliding face re-fires its observer once per display frame. Observed,
+/// each of those writes was an `@Observable` mutation with registrar bookkeeping — dozens of cards at
+/// the display's refresh rate, invalidating the strip on top of the reflow's own render work (an A/B
+/// on 2026-07-31 confirmed it: removing the card-face registration alone made dragging visibly
+/// smoother). Unobserved, a registration is a dictionary store and nothing else.
+///
+/// The alternative — suppressing the writes while a drag is in flight — was rejected: a suppressed
+/// write never replays if that card's geometry does not change again after the drag ends, which
+/// leaves the band and the arrows navigating by stale rectangles. The frames must stay live; only
+/// their observation had to go.
 @MainActor
-@Observable
 final class MarqueeTargetRegistry {
 
     private(set) var targets: [ItemID: MarqueeTarget] = [:]