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/view/view_mode_model.h

@@ -12,6 +12,8 @@
 #include <string>
 #include <vector>
 
+#include "core/view/solo_cache.h"
+
 namespace reasampler {
 
 // Stable seed-mode ids. Arrange is the default home for untagged leaves.
@@ -291,6 +293,8 @@ public:
     const MembershipIndex&  membership() const { return membership_; }
     LaneOwnershipIndex&        lanes()       { return lanes_; }
     const LaneOwnershipIndex&  lanes() const { return lanes_; }
+    view::SoloCache&           soloCache()       { return soloCache_; }
+    const view::SoloCache&     soloCache() const { return soloCache_; }
 
     const std::string& activeModeId() const { return activeModeId_; }
     // Returns false (no change) if the id is not registered.
@@ -302,8 +306,9 @@ public:
     const TrackSnapshot* snapshot(const std::string& guid) const;
     const std::map<std::string, TrackSnapshot>& snapshots() const { return snapshots_; }
 
-    // Drops every snapshot whose GUID is NOT in `liveGuids`. Returns the count
-    // removed.
+    // Drops every snapshot whose GUID is NOT in `liveGuids`, and prunes the solo
+    // cache the same way (see SoloCache::reconcile). Returns the count of
+    // SNAPSHOTS removed — the solo cache's own count is available from it directly.
     //
     // Snapshots are pruned, membership is not: a parked track's snapshot is
     // dead weight once the track is deleted (can never restore; a reused GUID
@@ -355,6 +360,7 @@ private:
     LaneOwnershipIndex lanes_;                            // (guid, laneKey) -> ownership
     std::string     activeModeId_;                       // always a registered id
     std::map<std::string, TrackSnapshot> snapshots_;     // guid -> pre-park snapshot
+    view::SoloCache soloCache_;                          // modeId -> guid -> raw I_SOLO
 };
 
 // Fixed-zero park plan for one leaf, offlining `fxCount` slots.