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.

tests/test_action_bar.cpp

@@ -229,6 +229,48 @@ static void testOverflowReserveHonouredAcrossWidths() {
     CHECK(sawReserveCostAButton);
 }
 
+// --- Reserve against the REAL panel spec: names the button and the exact width -----
+
+// roundSpec() above uses clusterGap=16 for hand-checkable pixel math; the panel's actual
+// bar (shell/panel/panel_state.h's kBarSpec) uses clusterGap=24. Reproduced literally here
+// because this test target is REAPER-free and cannot include that shell header -- keep
+// the two in sync by hand if either spec ever changes.
+static ActionBarSpec realBarSpec() {
+    ActionBarSpec s;
+    s.buttonWidth = 108;
+    s.buttonGap = 4;
+    s.clusterGap = 24;
+    s.sidePad = 8;
+    s.verticalInset = 3;
+    return s;
+}
+
+// At the real spec plus the real 40 px reserve (28 + 2*6, panel_state.h's kMenuBtnSpec --
+// all default-constructed here since its defaults already match), 764 is the narrowest
+// band that still shows all six buttons; one pixel narrower drops Insert as FX (flat index
+// 5, the trailing Placement button) alone into overflow, leaving its five neighbours intact.
+static void testReserveDropsInsertAsFxAtRealWidth() {
+    const auto clusters = inventory();
+    const ActionBarSpec spec = realBarSpec();
+    const MenuButtonSpec menuSpec;
+    const int reserve = menuButtonReserve(MenuBarRect{0, 0, 900, 34}, menuSpec);
+    CHECK(reserve == 40);
+
+    const int allSixWidth = 764;
+    {
+        ActionBarRect action{0, 0, allSixWidth - reserve, 34};
+        const auto slots = computeBarSlots(action, clusters, spec);
+        CHECK(slots.size() == 6);
+        CHECK(slots.back().index == 5);  // Insert as FX still visible
+    }
+    {
+        ActionBarRect action{0, 0, allSixWidth - 1 - reserve, 34};
+        const auto slots = computeBarSlots(action, clusters, spec);
+        CHECK(slots.size() == 5);        // Insert as FX alone dropped
+        CHECK(slots.back().index == 4);  // Insert Conform is now the trailing visible button
+    }
+}
+
 // --- Degenerate --------------------------------------------------------------
 
 static void testDegenerate() {
@@ -356,6 +398,7 @@ int main() {
     testOverflowDropsTrailingWhole();
     testTooNarrowForAny();
     testOverflowReserveHonouredAcrossWidths();
+    testReserveDropsInsertAsFxAtRealWidth();
     testDegenerate();
     testHitTestHitsButtons();
     testHitTestGapsAreMisses();