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.

backend/internal/inttest/social_test.go

@@ -70,7 +70,7 @@ func newGameWithSeats(t *testing.T, n int) (uuid.UUID, []uuid.UUID) {
 
 // TestFriendRequestToRobotStaysPending checks a friend request to a robot is accepted as
 // pending rather than blocked: robots no longer block friend requests, so the request
-// just sits unanswered and later expires — mirroring a human who ignores it (Stage 17).
+// just sits unanswered and later expires — mirroring a human who ignores it.
 func TestFriendRequestToRobotStaysPending(t *testing.T) {
 	ctx := context.Background()
 	svc := newSocialService()
@@ -342,7 +342,7 @@ func TestChatRejectsBadContent(t *testing.T) {
 	}
 }
 
-// TestChatOnlyOnYourTurn checks chat is allowed only on the sender's own turn (Stage 17):
+// TestChatOnlyOnYourTurn checks chat is allowed only on the sender's own turn:
 // the player to move can post, the waiting player gets ErrChatNotYourTurn.
 func TestChatOnlyOnYourTurn(t *testing.T) {
 	ctx := context.Background()
@@ -383,7 +383,7 @@ func TestNudgeRulesAndRateLimit(t *testing.T) {
 }
 
 // TestNudgeCooldownResetsOnAction checks the nudge cooldown clears once the player has
-// acted (moved or chatted) since their last nudge, even within the hour (Stage 17).
+// acted (moved or chatted) since their last nudge, even within the hour.
 func TestNudgeCooldownResetsOnAction(t *testing.T) {
 	ctx := context.Background()
 	svc := newSocialService()
@@ -413,7 +413,7 @@ func TestNudgeCooldownResetsOnAction(t *testing.T) {
 }
 
 // TestListOutgoingRequests checks the requester-side list that backs the in-game "add to
-// friends" item (Stage 17): a pending request shows for the requester only; an accepted one
+// friends" item: a pending request shows for the requester only; an accepted one
 // clears (it is a friendship now); a declined one stays (cannot be re-sent, so it reads as
 // still "sent"); a lazily expired pending one drops (it may be re-sent).
 func TestListOutgoingRequests(t *testing.T) {
@@ -469,7 +469,7 @@ func TestListOutgoingRequests(t *testing.T) {
 }
 
 // TestRespondPublishesToRequester checks that answering a request notifies the original
-// requester over the live channel (Stage 17): accept -> friend_added, decline ->
+// requester over the live channel: accept -> friend_added, decline ->
 // friend_declined, so a game screen watching that opponent re-derives its friend state.
 func TestRespondPublishesToRequester(t *testing.T) {
 	ctx := context.Background()
@@ -503,7 +503,7 @@ func TestRespondPublishesToRequester(t *testing.T) {
 }
 
 // TestNudgeRoutedByGameLanguage checks a nudge's out-of-app push carries the game's language, so
-// it is delivered by the game's bot rather than the recipient's last-login bot (Stage 17).
+// it is delivered by the game's bot rather than the recipient's last-login bot.
 func TestNudgeRoutedByGameLanguage(t *testing.T) {
 	ctx := context.Background()
 	svc := newSocialService()
@@ -528,7 +528,7 @@ func TestNudgeRoutedByGameLanguage(t *testing.T) {
 	}
 }
 
-// TestAdminListMessages checks the admin moderation list (Stage 17): real messages only
+// TestAdminListMessages checks the admin moderation list: real messages only
 // (nudges excluded), the game / sender pins, the sender glob masks, and the source label.
 func TestAdminListMessages(t *testing.T) {
 	ctx := context.Background()