refactor(web): remove now-unused ViewerAsset proximity $derived

With day-tier boundaries handling viewport classification (previous two
commits), the per-asset #viewportProximity $derived and isInOrNearViewport
getter are no longer read by anyone. Remove them, along with the
calculateViewerAssetViewportProximity helper they used, and the back-pointer
to TimelineDay that only existed to feed the derive.

Net effect at scale: one $derived per ViewerAsset deleted. For a library
with N assets, that's N $derived instances and their dependency-tracking
metadata no longer allocated. The work that those derives did is now
performed lazily, O(log N) per day, by binary search in updateAssetBoundaries.

Change-Id: I77f5eee5a4a5ebb7968e7f87955dcd516a6a6964

web/src/lib/managers/timeline-manager/internal/intersection-support.svelte.ts

@@ -54,16 +54,3 @@ export function updateTimelineMonthViewportProximity(timelineManager: TimelineMa
   }
 }
 
-export function calculateViewerAssetViewportProximity(
-  timelineManager: TimelineManager,
-  positionTop: number,
-  positionHeight: number,
-) {
-  const headerHeight = timelineManager.headerHeight;
-  return calculateViewportProximity(
-    positionTop,
-    positionTop + positionHeight,
-    timelineManager.visibleWindow.top - headerHeight,
-    timelineManager.visibleWindow.bottom + headerHeight,
-  );
-}

web/src/lib/managers/timeline-manager/timeline-month.svelte.ts

@@ -254,7 +254,7 @@ export class TimelineMonth {
       addContext.newTimelineDays.add(timelineDay);
     }
 
-    const viewerAsset = new ViewerAsset(timelineDay, timelineAsset);
+    const viewerAsset = new ViewerAsset(timelineAsset);
     timelineDay.viewerAssets.push(viewerAsset);
     addContext.changedTimelineDays.add(timelineDay);
   }

web/src/lib/managers/timeline-manager/viewer-asset.svelte.ts

@@ -1,36 +1,12 @@
 import type { CommonPosition } from '$lib/utils/layout-utils';
-import {
-  ViewportProximity,
-  calculateViewerAssetViewportProximity,
-  isInOrNearViewport,
-} from './internal/intersection-support.svelte';
-import type { TimelineDay } from './timeline-day.svelte';
 import type { TimelineAsset } from './types';
 
 export class ViewerAsset {
-  readonly #group: TimelineDay;
-
-  #viewportProximity = $derived.by(() => {
-    if (!this.position) {
-      return ViewportProximity.FarFromViewport;
-    }
-
-    const store = this.#group.timelineMonth.timelineManager;
-    const positionTop = this.#group.absoluteTimelineDayTop + this.position.top;
-
-    return calculateViewerAssetViewportProximity(store, positionTop, this.position.height);
-  });
-
-  get isInOrNearViewport() {
-    return isInOrNearViewport(this.#viewportProximity);
-  }
-
   position: CommonPosition | undefined = $state.raw();
   asset: TimelineAsset = $state() as TimelineAsset;
   id: string = $derived(this.asset.id);
 
-  constructor(group: TimelineDay, asset: TimelineAsset) {
-    this.#group = group;
+  constructor(asset: TimelineAsset) {
     this.asset = asset;
   }
 }