R6(a): de-stage code, docs, READMEs; split stage6_test

Mechanical, behaviour-preserving removal of Stage N / TODO-N / phase (RN)
references from comments, doc-comments, service READMEs, the current-state docs
(ARCHITECTURE, FUNCTIONAL+_ru, TESTING, UI_DESIGN), config-file comments, and the
.fbs/.proto schema comments. PLAN.md / PRERELEASE.md / CLAUDE.md keep the stage
history.

- Rename the only stage-named identifiers: registerStage8 -> registerSocialOps,
  registerStage11 -> registerLinkOps (gateway transcode).
- Split stage6_test.go: TestEmailLoginFlow -> email_test.go,
  TestGuestAutoMatchLeavesNoStats (+ provisionGuest) -> account_test.go.
- Regenerated proto bindings (push.pb.go, telegram_grpc.pb.go) from the de-staged
  .proto comments; FB Go/TS bindings unchanged (flatc strips schema comments).

go build/vet/gofmt clean across modules; integration typecheck and pnpm check green.

ui/src/lib/draft.ts

@@ -1,5 +1,5 @@
 // Draft (client-side composition) serialization, kept pure for unit tests. The server stores
-// the JSON opaquely (Stage 17); only the client interprets {rack_order, board_tiles}. The rack
+// the JSON opaquely; only the client interprets {rack_order, board_tiles}. The rack
 // order is a comma-joined permutation of the server rack's indices, in the player's visual
 // order; the board tiles are the tiles laid but not yet submitted.