perf: virtual windowing, markdown memoisation, and search debounce

- Terminal: virtual windowing renders max 150 lines, loads 50 older on
  scroll-up with scroll position compensation; auto-advances window
  forward during auto-scroll so old DOM nodes are unloaded continuously
- Markdown: two-stage derived rendering separates expensive parse step
  (marked + hljs + spoilers, runs on content change) from cheap search
  highlight step (runs on query change only)
- Achievements: fix double Object.keys() call in derived store
- Terminal: 150ms debounce on search query to reduce redundant updates
- Tests: add Markdown.test.ts for processSpoilers and highlightSearchMatches;
  extend Terminal.test.ts with virtual windowing helper coverage

src/lib/stores/achievements.ts

@@ -1471,11 +1471,14 @@ export const achievementsByRarity = derived(achievementsStore, ($store) => {
   return byRarity;
 });
 
-export const achievementProgress = derived(achievementsStore, ($store) => ({
-  unlocked: $store.totalUnlocked,
-  total: Object.keys($store.achievements).length,
-  percentage: Math.round(($store.totalUnlocked / Object.keys($store.achievements).length) * 100),
-}));
+export const achievementProgress = derived(achievementsStore, ($store) => {
+  const total = Object.keys($store.achievements).length;
+  return {
+    unlocked: $store.totalUnlocked,
+    total,
+    percentage: Math.round(($store.totalUnlocked / total) * 100),
+  };
+});
 
 // Initialize achievement listener
 export async function initAchievementsListener() {