feat(web): add useDelayed rune helper

Boolean mirror that flips true only after the source stays true for
delayMs. Used to hide the skeleton on sub-100ms cache hits so
back-navigation feels instant.

web/src/lib/utils/useDelayed.svelte.ts

@@ -0,0 +1,46 @@
+// Returns a rune-wrapped boolean that mirrors the source getter, but only
+// flips to true after the source has been true for delayMs continuously.
+// Flips back to false immediately when the source becomes false.
+//
+// Used to suppress flash-of-skeleton on cache-hit navigation: skeletons
+// only render for loads that take longer than ~100ms.
+export function useDelayed(
+  source: () => boolean,
+  delayMs = 100
+): { readonly value: boolean } {
+  let delayed = $state(false);
+  let timeout: ReturnType<typeof setTimeout> | null = null;
+
+  // Schedule the initial timeout synchronously so tests that advance fake
+  // timers before the first reactive flush still see the correct value.
+  if (source()) {
+    timeout = setTimeout(() => {
+      delayed = true;
+      timeout = null;
+    }, delayMs);
+  }
+
+  $effect(() => {
+    const v = source();
+    if (timeout) {
+      clearTimeout(timeout);
+      timeout = null;
+    }
+    if (v) {
+      timeout = setTimeout(() => {
+        delayed = true;
+        timeout = null;
+      }, delayMs);
+    } else {
+      delayed = false;
+    }
+    return () => {
+      if (timeout) {
+        clearTimeout(timeout);
+        timeout = null;
+      }
+    };
+  });
+
+  return { get value() { return delayed; } };
+}