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/CLAUDE.md

@@ -95,7 +95,7 @@ L7 sub-pass, 2026-07-27):
 - `theme` — pure palette module: role→color mapping, REAPER-grey neutral ladder + the pastel accent system, the keyboard strip's spectral ramp, WCAG contrast-floor helpers + `compositeOver` (the effective color of a translucent fill, so alpha overlays are testable). Only the ramp's MID stop is its own constant; lo/hi are still aliases of `accent/primary`/`accent/tertiary`, so a categorical accent move CAN still reorder the ramp — `testSpectralRampLuminanceIsMonotonic` is the build-time catch, not the structure.
 - `component_geometry` — pure button/slider/list-row geometry + hover hit-test helpers.
 - `action_bar` — pure task-grouped action-bar layout/hit-test: clusters (Capture / Placement / Maintenance / Tagging / Switching).
-- `footer_bar` — pure footer layout/hit-test: `[Arrange|Design]` mode-toggle geometry, Tail button, and Prune placement.
+- `footer_bar` — pure footer layout/hit-test: `[Arrange|Design]` mode-toggle geometry, Tail button, and Prune placement, plus `modeSegmentEnabled` — the mode segment's live/dead predicate under the playback gate (the transport bool is passed IN, so this stays REAPER-free).
 - `overflow_menu` — pure overflow-menu-button geometry/reserve/hit-test for the top-toolbar More (⋯) button.
 - `mode_enable` — pure opposite-mode enablement predicate: given the active mode, computes per-button live/disabled state for the four Item/Track × Arrange/Design tag buttons.
 - `tooltip` — pure tooltip placement + prefix-strip: strips the `ReaSampler:` display prefix from the registered action phrase; width clamped to the client rect.

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

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