Ψ-W1-T2: disjoint per-mode solo surfaces and a playback-gated mode switch

Solo is cached, cleared and replayed per mode on a real switch only; the switch is refused visibly while the transport runs. The footer segment now routes through the activate actions, so a panel switch finally persists.

src/shell/panel/panel_state.h

@@ -130,6 +130,7 @@ using ui::hitTestPruneButton;
 using ui::hitTestSlot;
 using ui::hitTestTabStrip;
 using ui::menuButtonReserve;
+using ui::modeSegmentEnabled;
 using ui::navigate;
 using ui::roleColor;
 using ui::stripActionPrefix;
@@ -291,6 +292,12 @@ struct PanelState {
     unsigned int hoverSinceTick = 0;
     bool tooltipShown = false;
 
+    // Polled on the OnTimer tick (REAPER exposes no transport-change callback): a mode
+    // switch is refused while the transport runs, so the footer's [Arrange|Design]
+    // segments paint disabled. Cached rather than read per paint AND per click so the
+    // pixel the user saw and the click they made cannot disagree within a tick.
+    bool modeSwitchBlocked = false;
+
     BankPanelFullHeight fullHeight = BankPanelFullHeight::Split;
 
     // The named bank the banks region shows — distinct from the active/capture-target