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.

pkg/proto/telegram/v1/telegram.proto

@@ -23,7 +23,7 @@ service Telegram {
   // ValidateLoginWidget verifies Telegram Login Widget authorization data (the web
   // sign-in flow, HMAC under SHA-256(bot_token)) and returns the authenticated
   // user. The gateway calls it during the link.telegram edge operation to attach a
-  // Telegram identity to an existing account (Stage 11).
+  // Telegram identity to an existing account.
   rpc ValidateLoginWidget(ValidateLoginWidgetRequest) returns (ValidateLoginWidgetResponse);
   // Notify delivers an out-of-app notification for a backend push event. The
   // gateway calls it only for a recipient with no live in-app stream (so the
@@ -32,11 +32,11 @@ service Telegram {
   // payload; unrenderable kinds are skipped (delivered=false).
   rpc Notify(NotifyRequest) returns (NotifyResponse);
   // SendToUser sends an arbitrary text message to one user through the bot the
-  // request selects by language (admin use, wired in Stage 10). delivered is false
+  // request selects by language (admin use). delivered is false
   // when the user has not started that bot.
   rpc SendToUser(SendToUserRequest) returns (SendResponse);
   // SendToGameChannel posts an arbitrary text message to the game channel of the
-  // bot the request selects by language (admin use, wired in Stage 10); the channel
+  // bot the request selects by language (admin use); the channel
   // ids live only in the connector configuration.
   rpc SendToGameChannel(SendToGameChannelRequest) returns (SendResponse);
 }