WIP(meteor3): boot progresses past linker + several async fixes

Past the client-bundle underscore linker (pin underscore@1.6.4) and now working
through the server async/await migration on Meteor 3.1 + Mongo 7:

- Removed fibers (imports/startup/server/fibers.js + import) — gone in Meteor 3.
- accounts/oauth.js: upsert -> upsertAsync (top-level await).
- migrations.js: startup async + await Migrations.migrateTo (async in 2.x).
  mongo7 marked at version 18 so historical up() bodies (still sync Mongo) don't
  run — prod data restores already-migrated. Documented.
- subsUnion.js: fully async (fetchAsync/findOneAsync/countAsync/upsertAsync,
  observeAsync with async callbacks).
- facts.js: dropped dead sync findOne.
- email.js: ostrio:mailer 2.5 is a default export (was named import); dropped
  removed static MailTime.Template (built-in default '{{{html}}}').

NEXT wall + remaining chain (multi-session):
1. aldeed:collection2@4.2.0 needs simpl-schema 3.x (app has 1.x) -> 'attachSchema
   is not a function'. Requires upgrading npm simpl-schema to 3.x and migrating
   all collection schemas (SimpleSchema.RegEx.Id etc.) across ~8 files.
2. Rest.js + helpers (countRealFires/firesUnion/whichAreFalsePositives/
   fireFromHash/subscriptionsInsert/upsertFalsePositive) -> *Async.
3. Patch vendored restivus to await async endpoint handlers.
4. methods/publications/comments to async; cron SyncedCron(quave)/Facts imports.
5. alanning:roles@1.2.10 + gadicohen:sitemaps@0.0.17 warn incompatible -> bump.
6. React 16 -> 18 render root.

Run with: NODE_OPTIONS='--dns-result-order=ipv4first --no-network-family-autoselection'
MONGO_URL='mongodb://localhost:27019/fuegos?replicaSet=rs0'

.meteor/packages

@@ -54,3 +54,4 @@ nspangler:autoreconnect
 quave:synced-cron
 # lmachens:kadira
 nimble:restivus@0.8.12
+underscore@1.6.4

.meteor/versions

@@ -120,7 +120,7 @@ themeteorchef:bert@1.1.0
 tmeasday:check-npm-versions@0.3.2
 tracker@1.3.4
 typescript@5.6.3
-underscore@1.6.2
+underscore@1.6.4
 url@1.3.5
 vjrj:piwik@0.3.1
 webapp@2.0.4