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/notify/events.go

@@ -15,11 +15,11 @@ import (
 // the game/social/lobby services emit events without importing the wire schema.
 
 // YourTurn announces to userID that it is their turn in game gameID, with the turn's nominal
-// deadline. opponentName, lastAction, lastWord and scoreLine enrich the out-of-app push (Stage
-// 17): the player who just moved, their move kind, the main word of a scoring play (empty
+// deadline. opponentName, lastAction, lastWord and scoreLine enrich the out-of-app push:
+// the player who just moved, their move kind, the main word of a scoring play (empty
 // otherwise) and the recipient-first running score line. Empty strings render the plain "your
 // turn" text. moveCount is the post-move count, which the client compares against its cached
-// game to detect a missed in-app move and fall back to a refetch (R4).
+// game to detect a missed in-app move and fall back to a refetch.
 func YourTurn(userID, gameID uuid.UUID, deadline time.Time, opponentName, lastAction, lastWord, scoreLine string, moveCount int) Intent {
 	b := flatbuffers.NewBuilder(128)
 	gid := b.CreateString(gameID.String())
@@ -41,9 +41,9 @@ func YourTurn(userID, gameID uuid.UUID, deadline time.Time, opponentName, lastAc
 
 // GameOver announces to userID that game gameID finished. result is the outcome from userID's
 // own perspective ("won"/"lost"/"draw") and scoreLine is the recipient-first final score; both
-// feed the out-of-app "game over" push (Stage 17). game is the final post-game summary (the
+// feed the out-of-app "game over" push. game is the final post-game summary (the
 // adjusted scores after rack penalties and the winner flag), so an in-app client settles the
-// finished game from the event without a refetch (R4).
+// finished game from the event without a refetch.
 func GameOver(userID, gameID uuid.UUID, result, scoreLine string, game GameSummary) Intent {
 	b := flatbuffers.NewBuilder(512)
 	gid := b.CreateString(gameID.String())
@@ -60,10 +60,10 @@ func GameOver(userID, gameID uuid.UUID, result, scoreLine string, game GameSumma
 }
 
 // OpponentMoved tells userID that move was just committed in game gameID, carrying it as a delta
-// the client applies to its cached game without a refetch (R4): move is the decoded play/pass/
+// the client applies to its cached game without a refetch: move is the decoded play/pass/
 // exchange, game is the post-move summary (per-seat scores, to_move, move_count, status) and
 // bagLen is the bag size after the draw. The seat/action/score/total scalars repeat the move's
-// summary for pre-R4 wire back-compat.
+// summary for wire back-compat.
 func OpponentMoved(userID, gameID uuid.UUID, move engine.MoveRecord, game GameSummary, bagLen int) Intent {
 	b := flatbuffers.NewBuilder(512)
 	gid := b.CreateString(gameID.String())
@@ -116,7 +116,7 @@ func Nudge(userID, gameID, fromUserID uuid.UUID) Intent {
 
 // MatchFound tells userID that game gameID, which they are seated in, has started (an auto-match
 // pairing or a robot substitution). state is the recipient's full initial view of the new game,
-// so the client navigates straight in from the event with no follow-up fetch (R4).
+// so the client navigates straight in from the event with no follow-up fetch.
 func MatchFound(userID, gameID uuid.UUID, state PlayerState) Intent {
 	b := flatbuffers.NewBuilder(512)
 	gid := b.CreateString(gameID.String())
@@ -131,7 +131,7 @@ func MatchFound(userID, gameID uuid.UUID, state PlayerState) Intent {
 // Notification is a lightweight "re-poll" signal to userID that something in their lobby
 // changed. kind is a sub-discriminator (NotifyFriendRequest, NotifyFriendAdded,
 // NotifyFriendDeclined, NotifyInvitation, NotifyGameStarted). It carries no payload; prefer the
-// enriched constructors below, which let the client update its lobby without a refetch (R4).
+// enriched constructors below, which let the client update its lobby without a refetch.
 func Notification(userID uuid.UUID, kind string) Intent {
 	b := flatbuffers.NewBuilder(32)
 	k := b.CreateString(kind)
@@ -143,7 +143,7 @@ func Notification(userID uuid.UUID, kind string) Intent {
 
 // NotificationAccount builds a lobby notification of one of the friend_* kinds carrying the
 // account it concerns (the requester, the new friend or the decliner), so the client updates its
-// requests/friends lists and the in-game "add friend" state without a refetch (R4).
+// requests/friends lists and the in-game "add friend" state without a refetch.
 func NotificationAccount(userID uuid.UUID, kind string, acc AccountRef) Intent {
 	b := flatbuffers.NewBuilder(128)
 	k := b.CreateString(kind)
@@ -157,7 +157,7 @@ func NotificationAccount(userID uuid.UUID, kind string, acc AccountRef) Intent {
 
 // NotificationGameStarted builds the NotifyGameStarted notification carrying the recipient's
 // initial view of the just-started invited game, so the client seeds its game cache and the
-// lobby list without a refetch (R4).
+// lobby list without a refetch.
 func NotificationGameStarted(userID uuid.UUID, state PlayerState) Intent {
 	b := flatbuffers.NewBuilder(512)
 	k := b.CreateString(NotifyGameStarted)
@@ -170,7 +170,7 @@ func NotificationGameStarted(userID uuid.UUID, state PlayerState) Intent {
 }
 
 // NotificationInvitation builds the NotifyInvitation notification carrying the new invitation,
-// so the client adds it to its lobby invitations list without a refetch (R4).
+// so the client adds it to its lobby invitations list without a refetch.
 func NotificationInvitation(userID uuid.UUID, inv InvitationSummary) Intent {
 	b := flatbuffers.NewBuilder(512)
 	k := b.CreateString(NotifyInvitation)