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/variants.ts

@@ -1,4 +1,4 @@
-// Game variants offered on New Game, and the Stage 15 gating of that choice by the
+// Game variants offered on New Game, and the gating of that choice by the
 // languages the sign-in service supports. Kept out of the .svelte screen so the
 // gating is unit-testable (the project's node-env Vitest layer).
 
@@ -14,8 +14,7 @@ export interface VariantOption {
 // ALL_VARIANTS lists every variant in display order. The labels are display names keyed by
 // the game's alphabet, not the interface language: the English-alphabet game is always
 // "Scrabble" and the Russian-alphabet Scrabble always "Скрэббл" (both unlocalized, so the
-// two never collide whatever the UI language); Erudit is localized "Erudite"/"Эрудит"
-// (Stage 17).
+// two never collide whatever the UI language); Erudit is localized "Erudite"/"Эрудит".
 export const ALL_VARIANTS: VariantOption[] = [
   { id: 'scrabble_en', label: 'new.english' },
   { id: 'scrabble_ru', label: 'new.russian' },