Fix Insert-as-FX review findings: id rename, selection-count drift, refusal tooltip, success msg, master-only, reserve test

Renames the permanent action id out of the placement family, makes the button's
painted state and pressed outcome share one selection count, surfaces
panel-known refusals in the tooltip, reports the target track on success,
distinguishes a master-only selection, and pins the overflow-reserve test to the
real bar spec.

src/core/ui/insert_fx_enable.cpp

@@ -4,10 +4,12 @@
 
 namespace reasampler::ui {
 
-InsertFxRefusal insertFxRefusal(int focusedCaptureCount, bool trackSelected) {
+InsertFxRefusal insertFxRefusal(int focusedCaptureCount, bool trackSelected,
+                                bool masterOnlySelected) {
     if (focusedCaptureCount <= 0) return InsertFxRefusal::NoCapture;
     if (focusedCaptureCount > 1) return InsertFxRefusal::MultiCapture;
-    if (!trackSelected) return InsertFxRefusal::NoTrack;
+    if (!trackSelected)
+        return masterOnlySelected ? InsertFxRefusal::MasterOnlySelected : InsertFxRefusal::NoTrack;
     return InsertFxRefusal::None;
 }
 
@@ -29,6 +31,9 @@ std::string insertFxRefusalMessage(InsertFxRefusal refusal) {
         case InsertFxRefusal::NoTrack:
             return "select a track first -- the instrument is added to the selected "
                    "track's FX chain";
+        case InsertFxRefusal::MasterOnlySelected:
+            return "select a track other than the master -- the master track cannot "
+                   "host the instrument";
     }
     return {};
 }