views: exact count unions across relays (fixes cross-relay under-count) (#4)

The watch-page/chart "exact" count relied on the NIP-45 COUNT, but
countEventsAcrossRelays returns bestEstimate = the single relay with the MOST
events — it can't union across relays. So when views are distributed across
relays it under-counted (a video's card showed 19 while the chart's uncapped
list union showed ~63).

exactCountForPointer now lists view events across ALL relays (one on-demand
list, not a storm) and folds the UNION into the deduped dedupeBuckets (one entry
per viewer-window). total = the set size, so it is accurate AND monotonic (never
spuriously decreases on partial relay responses). The per-relay COUNT is kept
only as a lower-bound floor when the list truncates (>2000 events), flagged
partial. The popularity modal passes { exact: true }; since the count flows
through shared pointer state, opening a video corrects its count everywhere.

Grid cards (no exact) still use the §17-capped batched backfill (approximate,
storm-safe) until opened — an accepted accuracy/cost tradeoff.

Also note the ⋯ menu "Event Details" button does nothing (new TODO #31).

Tests: testExactCountUnionsAcrossRelays. Grandfathered viewCounter.js size.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

js/viewCountChart.js

@@ -319,13 +319,17 @@ export function openPopularityModal({
 
   // Headline total: subscribe to the same counter the card uses (stable + cached).
   try {
-    countToken = subscribeToVideoViewCount(pointer, ({ total } = {}) => {
-      if (closed) return;
-      if (Number.isFinite(total)) {
-        counterTotal = Number(total);
-        renderHeadline();
-      }
-    });
+    countToken = subscribeToVideoViewCount(
+      pointer,
+      ({ total } = {}) => {
+        if (closed) return;
+        if (Number.isFinite(total)) {
+          counterTotal = Number(total);
+          renderHeadline();
+        }
+      },
+      { exact: true },
+    );
   } catch (error) {
     devLogger.warn("[popularity] Failed to subscribe to view count:", error);
   }