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.h

@@ -61,4 +61,11 @@ FooterBarLayout computeFooterBar(const FooterRect& footer, const FooterBarSpec&
 // a passive readout, never a control). Half-open bounds match computeFooterBar.
 FooterHit hitTestFooterBar(int px, int py, const FooterBarLayout& layout);
 
+// Whether one [Arrange|Design] segment is live. A mode switch is refused while the project
+// plays or records (shell/view::transportBlocksModeSwitch), so an unreachable segment must READ
+// dead before the click, not merely refuse on it. The ACTIVE segment stays live regardless: it
+// fires a reapply, which is never gated, and dimming the mode you are already in would read as
+// "this mode is unavailable" rather than "you cannot leave it right now".
+bool modeSegmentEnabled(bool isActiveSegment, bool transportRunning);
+
 } // namespace reasampler::ui