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/profileValidation.ts

@@ -14,7 +14,7 @@ export const maxAwayMinutes = 12 * 60;
 
 // Unicode letters joined by single space / "." / "_" separators, where a "." or "_"
 // may be followed by a single space. No leading separator and no adjacent separators
-// except "<dot|underscore> <space>"; a single trailing "." is allowed (Stage 17). Same
+// except "<dot|underscore> <space>"; a single trailing "." is allowed. Same
 // rule as the Go displayNameRe.
 const displayNameRe = /^\p{L}+(?:(?:[._] ?| )\p{L}+)*\.?$/u;