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/game/Chat.svelte

@@ -15,7 +15,7 @@
     messages: ChatMessage[];
     myId: string;
     busy: boolean;
-    // Chat and nudge are mutually exclusive by turn (Stage 17): on the player's own turn the
+    // Chat and nudge are mutually exclusive by turn: on the player's own turn the
     // message field + send are shown (and nudging makes no sense — there is no one to
     // hurry); on the opponent's turn only the nudge button shows. While the hourly nudge
     // cooldown is active the nudge is disabled with an "awaiting reply" caption.
@@ -72,7 +72,7 @@
     gap: 10px;
     /* Fill the chat screen; the list scrolls and the input pins to the bottom. The screen
        fits the visual viewport (--vvh), so an open keyboard simply shrinks it and the input
-       stays visible — no modal relayout, no page jump (Stage 17). */
+       stays visible — no modal relayout, no page jump. */
     flex: 1;
     min-height: 0;
     padding: 10px var(--pad);