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/view/view_solo.cpp

@@ -0,0 +1,47 @@
+// See view_solo.h. Compiled into the reaper_reasampler module; includes
+// reaper_plugin_functions.h without REAPERAPI_IMPLEMENT (main.cpp owns that).
+
+#include "shell/view/view_solo.h"
+
+#include <map>
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_GetMediaTrackInfo_Value
+#define REAPERAPI_WANT_SetMediaTrackInfo_Value
+#include "reaper_plugin_functions.h"
+
+namespace reasampler {
+
+namespace {
+// SDK: int, 0=not soloed, 1=soloed, 2=soloed in place, 5=safe soloed,
+// 6=safe soloed in place. Documented under SetMediaTrackInfo_Value with no
+// read-only marker (unlike B_RECMON_IN_EFFECT in the same list), so it is settable.
+constexpr const char* kSoloParm = "I_SOLO";
+}  // namespace
+
+std::vector<view::TrackSolo> readTrackSolos(const TrackHandles& handles) {
+    std::vector<view::TrackSolo> live;
+    live.reserve(handles.size());
+    for (const auto& [guid, tr] : handles) {
+        if (!tr) continue;
+        live.push_back(view::TrackSolo{
+            guid, static_cast<int>(GetMediaTrackInfo_Value(tr, kSoloParm))});
+    }
+    return live;
+}
+
+void applySoloOps(const TrackHandles& handles, const std::vector<view::SoloOp>& ops) {
+    if (ops.empty()) return;  // the common case: nothing soloed, no project write
+
+    std::map<std::string, int> byGuid;
+    for (const view::SoloOp& op : ops) byGuid[op.guid] = op.value;
+
+    for (const auto& [guid, tr] : handles) {
+        if (!tr) continue;
+        auto it = byGuid.find(guid);
+        if (it == byGuid.end()) continue;
+        SetMediaTrackInfo_Value(tr, kSoloParm, static_cast<double>(it->second));
+    }
+}
+
+} // namespace reasampler