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/core/ui/footer_bar.cpp

@@ -64,4 +64,8 @@ FooterHit hitTestFooterBar(int px, int py, const FooterBarLayout& layout) {
     return FooterHit::None;
 }
 
+bool modeSegmentEnabled(bool isActiveSegment, bool transportRunning) {
+    return isActiveSegment || !transportRunning;
+}
+
 } // namespace reasampler::ui