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/screens/Lobby.svelte

@@ -62,7 +62,7 @@
     return `${me?.score ?? 0} : ${opp.join(', ')}`;
   }
 
-  // Hiding a finished game (Stage 17). The delete action sits behind each finished row and is
+  // Hiding a finished game. The delete action sits behind each finished row and is
   // revealed by swiping the row left (touch) or tapping its kebab (any pointer); the action is
   // per-account and irreversible. Only one row is revealed at a time.
   let revealedId = $state<string | null>(null);
@@ -277,13 +277,13 @@
     user-select: none;
   }
   /* Game rows are a compact, flat list: no per-card frame, a hairline divider between
-     consecutive rows (Stage 17). */
+     consecutive rows. */
   .list {
     display: flex;
     flex-direction: column;
   }
   /* Each finished row can slide left to reveal a delete action sitting behind it; the row's
-     own opaque background hides that action until revealed (Stage 17). */
+     own opaque background hides that action until revealed. */
   .rowwrap {
     position: relative;
     overflow: hidden;