Add scene fade transitions, gacha pull reveal, and modal/hover animations

Menu <-> combat scene loads now fade through black instead of hard-cutting,
gacha pulls reveal reward cards with a staggered stagger-in and a rarity
flash instead of a static text block, starter selection and root-credit
exchange modals fade+scale in/out instead of popping via display:none, and
menu/collection/gacha rows now transition on hover/select instead of
snapping. All new motion respects the existing reduced-motion setting.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015hiJi3irEg1RegCXRaQDcu

Assets/Resources/CombatScene.uss

@@ -46,6 +46,37 @@
     background-color: var(--kp-bg);
     color: var(--kp-text);
     font-size: 13px;
+    opacity: 1;
+    transition-property: opacity;
+    transition-duration: 220ms;
+}
+
+.combat-root.scene-fade-hidden {
+    opacity: 0;
+}
+
+.combat-root.reduced-motion {
+    transition-duration: 0ms;
+}
+
+.scene-fade-curtain {
+    position: absolute;
+    left: 0;
+    right: 0;
+    top: 0;
+    bottom: 0;
+    background-color: rgb(0, 0, 0);
+    opacity: 0;
+    transition-property: opacity;
+    transition-duration: 200ms;
+}
+
+.scene-fade-curtain.scene-fade-visible {
+    opacity: 1;
+}
+
+.reduced-motion .scene-fade-curtain {
+    transition-duration: 0ms;
 }
 
 .combat-status-bar {