perf : deferred slide rendering + sequential loading by proximity. related to #55

Only render the active slide initially. After its critical media (videos)
fires canplaythrough, progressively render remaining slides by distance.
JSON loading is now sequential by proximity instead of all-parallel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/router/index.js

@@ -76,10 +76,16 @@ async function loadSlide(path) {
   }
 }
 
-function loadAllSlidesInBackground(exceptPath) {
-  slides.all
-    .filter((s) => s.path !== exceptPath)
-    .forEach((s) => loadSlide(s.path));
+async function loadAllSlidesInBackground(exceptPath) {
+  const activeIdx = slides.getIndexByPath(exceptPath)
+  const remaining = slides.all
+    .map((s, i) => ({ path: s.path, distance: Math.abs(i - activeIdx) }))
+    .filter(s => s.path !== exceptPath)
+    .sort((a, b) => a.distance - b.distance)
+
+  for (const { path } of remaining) {
+    await loadSlide(path)
+  }
 }
 
 export function slideTo(path, { skipHistory = false } = {}) {