feat(usage): count what happened away from a record's own project — the readout #3735 needs

Milestone 385 step 8 (#3735 "a lesson is recalled on a project it was not
written on") is defined by opened-on-another-project. The reader's project has
been recorded on every usage event since 0c8e109, but nothing compared it with
the record's own, so the criterion was still unreadable.

- note_usage.usage_for_notes: a second aggregate in the same session joins
  notes and counts surfaced_away_count / pulled_away_count. Counted only where
  both projects are known and differ; ranked surfacings only (#2477). The
  first aggregate is untouched, so events on deleted notes still count.
- empty_usage carries both keys zero-filled; every door that attaches usage
  (list_lessons, get_lesson, snippets, knowledge) gets them through
  attach_usage.
- UsageBadge tooltip says "On other projects: surfaced N×, opened M×" when it
  happened, and nothing when it did not.
- Tests: the mocked split test feeds both aggregates; a unit test for the
  away counters; a real-Postgres test that home, unreported and ambient
  events are all left out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/UsageBadge.vue

@@ -42,9 +42,15 @@ const title = () => {
     ? `Last opened ${new Date(u.last_pulled_at).toLocaleDateString()}.`
     : "Never opened.";
   const verdict = isDeadWeight() ? ` ${props.deadWeightAdvice}` : "";
+  // Said only when it happened: "0× elsewhere" on every row would read as a
+  // finding about records that simply have not been near another project.
+  const awaySurfaced = u.surfaced_away_count ?? 0;
+  const away = awaySurfaced
+    ? ` On other projects: surfaced ${awaySurfaced}×, opened ${u.pulled_away_count ?? 0}×.`
+    : "";
   return (
     `Surfaced to an agent ${u.surfaced_count}×, opened in full ` +
-    `${u.pull_count}×. ${last}${verdict}`
+    `${u.pull_count}×.${away} ${last}${verdict}`
   );
 };
 </script>