meteor3: Comments methods and mail fan-out to async

comments.insert/edit/remove/like/dislike use *Async collection APIs;
requireOwner/toggle helpers are async; onCommentAdd iterates users with
forEachAsync. subscriptions.* methods were already async. REST smoke stays
byte-identical (12/12).

UPGRADE.md

@@ -236,6 +236,11 @@ MONGO_CONTAINER=tcef-mongo7 MONGO_SHELL=mongosh MONGO_PORT=27019 ./smoke/smoke.s
 - Note: the module-level `new Counter(...)` (natestrauser:publish-performant-counts)
   behind `falsePositivesTotal` boots fine; the publication itself is unused by
   the current UI — watch it if re-enabled.
+- ✅ **Comments methods → async** (`comments.insert/edit/remove/like/dislike`):
+  `findOne/insert/update/remove` → `*Async`, helpers (`requireOwner`, `toggle`)
+  async, `users.findOneAsync` for the username. `onCommentAdd` mail fan-out:
+  `users.find().forEach` → `forEachAsync`. Still fire-and-forget from the
+  insert. Comments UI staging verification pending (pre-existing note below).
 
 ### Still TODO before the web (not just REST) is fully deployable on 3.x
 - **React 16 → 18** render root + verify ancient react-* libs.