docs(UPGRADE): 3.x groundwork on meteor3-wip — Mongo 7 docker, 3.1 resolved, blockers

UPGRADE.md

@@ -197,6 +197,61 @@ When Mongo is ≥ 4.4 and the notifications service is live in prod, the remaini
 server code (collections/methods/publications `*Async`, no Fibers), React 18
 render root, and re-run the smoke test.
 
+## 3.x groundwork — done on branch `meteor3-wip` (in progress, not deployable)
+
+To de-risk the eventual 3.x jump, the migration was driven as far as it goes
+against a **modern dockerized MongoDB** (the "mongo superior" decision: give
+fuegos its own Mongo 7 instead of the shared rsmain 3.2, which is what actually
+unblocks Meteor 3's driver). This lives on `meteor3-wip`; `meteor3-upgrade` stays
+at the deployable Meteor 2.5.
+
+**Modern Mongo (local, reproducible):**
+```bash
+docker run -d --name tcef-mongo7 -p 27019:27019 mongo:7 --replSet rs0 --port 27019 --bind_ip_all
+docker exec tcef-mongo7 mongosh --port 27019 --quiet --eval 'rs.initiate({_id:"rs0",members:[{_id:0,host:"localhost:27019"}]})'
+export MONGO_URL="mongodb://localhost:27019/fuegos?replicaSet=rs0"
+# meteor-tool node 22 needs this to reach warehouse.meteor.com (else Happy-Eyeballs
+# picks an unreachable IPv6):
+export NODE_OPTIONS="--dns-result-order=ipv4first --no-network-family-autoselection"
+```
+
+**Reached `METEOR@3.1` with all package-version conflicts resolved.** Dead
+packages swapped for Meteor-3 equivalents:
+
+| Old (mongo@1.x / dead) | New |
+|---|---|
+| `mongo-livedata` | removed (deprecated alias) |
+| `facts` | `facts-base` |
+| `saucecode:timezoned-synced-cron` | `quave:synced-cron` |
+| `aldeed:collection2-core` | `aldeed:collection2@4.0.4` |
+| `mys:fonts` (pinned caching-compiler@1.0.0) | removed |
+| `fourseven:scss@4.14.1` (node-sass, fails on node 22) | `@5.0.0` (Dart Sass) + `sass` npm |
+| `nimble:restivus` (vendored) | accounts-password loosened to `2.0.0 \|\| 3.0.0` |
+
+SCSS then compiles (Dart Sass; only `@import`/`lighten`/`darken` deprecation
+warnings — cosmetic debt: migrate to `@use` + `color.adjust`).
+
+**Blocked at the client-bundle linker:** `Runtime is not available, but it uses
+features needing the runtime: underscore` — one of the old client atmosphere
+packages (bert / blaze-react / cookie-consent / meteor-status / bootstrap-4 /
+etc.) needs replacing for Meteor 3's linker. Beyond that, the **server
+async/await migration is the main remaining effort** (unavoidable for 3.x):
+
+- `Rest.js` + its helpers (`countRealFires`, `firesUnion`, `whichAreFalsePositives`,
+  `fireFromHash`, `subscriptionsInsert`, `upsertFalsePositive`) use sync Mongo
+  (`findOne`, `find().fetch()/.count()`, `insert/update/remove/upsert`) → must
+  become `*Async`.
+- The vendored **restivus must be patched to `await`** async endpoint handlers
+  (its `_callEndpoint` uses the return value synchronously).
+- `methods.js`/`publications.js` across the app, `subsUnion.js`, `migrations.js`,
+  the Comments feature, and startup files → async.
+- `cron.js` `SyncedCron` import (quave export) and `facts.js` `Facts` import
+  (facts-base export).
+- React 16 → 18 render root (`createRoot`) in `startup/client/index.js`.
+
+This is a multi-hour migration; the smoke test (now `mongosh`-capable, see
+`smoke/smoke.sh` on that branch) is the gate. Resume on `meteor3-wip`.
+
 ## Dependency debt (tracked, to resolve in the noted escala)
 
 | Dep | From | Target | Status |