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/server/handlers_user.go

@@ -27,7 +27,7 @@ func (s *Server) handleProfile(c *gin.Context) {
 }
 
 // submitPlayRequest places tiles in a direction on the player's turn. Each tile's Letter
-// is a wire alphabet index (Stage 13); for a blank it is the designated letter's index.
+// is a wire alphabet index; for a blank it is the designated letter's index.
 type submitPlayRequest struct {
 	Dir   string `json:"dir"`
 	Tiles []struct {
@@ -39,7 +39,7 @@ type submitPlayRequest struct {
 }
 
 // tilesFromRequest maps a play/evaluate request's index-addressed tiles to engine tile
-// records for the game's variant (Stage 13: a placed blank carries its designated letter's
+// records for the game's variant (a placed blank carries its designated letter's
 // index with Blank set). An out-of-range index surfaces as engine.ErrIllegalPlay (HTTP 400).
 func tilesFromRequest(variant engine.Variant, req submitPlayRequest) ([]engine.TileRecord, error) {
 	tiles := make([]engine.TileRecord, 0, len(req.Tiles))
@@ -94,7 +94,7 @@ func (s *Server) handleSubmitPlay(c *gin.Context) {
 }
 
 // handleGameState returns the player's view of a game.
-// handleHideGame hides a finished game from the caller's own lobby list (Stage 17).
+// handleHideGame hides a finished game from the caller's own lobby list.
 func (s *Server) handleHideGame(c *gin.Context) {
 	uid, ok := userID(c)
 	if !ok {