fix(adom): parity fixes from the runed audit (IsIdle, IsInViewport)

A line-by-line audit of the ports against runed's original source found two
faithfulness regressions in unused F3 utilities:

- IsIdle seeded `lastActive` to 0 (epoch) instead of the construction
  timestamp, so `Date.now() - lastActive` read ~56 years until first activity.
  Now `$state(Date.now())`, matching runed.
- IsInViewport passed `threshold: undefined`, which the IntersectionObserver
  spec coerces to 0 (fires at a single pixel). runed defaults it to 0.1;
  restored (and rootMargin -> '0px').

Guarded IsIdle's baseline in its test. check=59.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/arts/adom/is-idle.svelte.test.ts

@@ -17,6 +17,8 @@ describe('IsIdle', () => {
 			flushSync();
 		});
 		expect(idle.current).toBe(false);
+		// lastActive is seeded to the construction time, not the epoch.
+		expect(idle.lastActive).toBeGreaterThan(0);
 
 		await vi.advanceTimersByTimeAsync(1000);
 		expect(idle.current).toBe(true);

src/arts/adom/is-idle.svelte.ts

@@ -32,7 +32,9 @@ export type IsIdleOptions = {
 
 export class IsIdle {
 	#current = $state(false);
-	#lastActive = $state(0);
+	// Baseline is the construction instant (matching runed), so `lastActive` is a
+	// plausible timestamp before any activity — not the epoch.
+	#lastActive = $state(Date.now());
 
 	constructor(options: IsIdleOptions = {}) {
 		const win = options.window ?? (isBrowser ? window : undefined);
@@ -44,7 +46,6 @@ export class IsIdle {
 		const detectVisibilityChanges = $derived(toValue(options.detectVisibilityChanges) ?? false);
 		const trackLastActive = options.trackLastActive ?? true;
 		this.#current = options.initialState ?? false;
-		this.#lastActive = 0;
 
 		const debouncedReset = debounce(
 			() => {

src/arts/adom/is-in-viewport.svelte.ts

@@ -37,7 +37,13 @@ export class IsInViewport {
 					}
 					this.#isInViewport = isIntersecting;
 				},
-				{ root: options.root, rootMargin: options.rootMargin, threshold: options.threshold }
+				{
+					root: options.root,
+					rootMargin: options.rootMargin ?? '0px',
+					// Match runed's default (0.1) — an unset threshold otherwise coerces to 0,
+					// firing at a single pixel instead of once ~10% is in view.
+					threshold: options.threshold ?? 0.1
+				}
 			);
 			observer.observe(el);
 			return () => observer.disconnect();