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.

gateway/internal/transcode/transcode.go

@@ -2,7 +2,7 @@
 // maps to a handler that decodes the FlatBuffers request payload, calls the
 // backend over REST, and encodes the FlatBuffers response. The registry is the
 // authoritative message_type catalog; new operations are added here following the
-// same pattern (PLAN.md Stage 6 vertical slice).
+// same vertical-slice pattern.
 package transcode
 
 import (
@@ -69,7 +69,7 @@ type Registry struct {
 }
 
 // TelegramValidator validates Telegram credentials via the connector side-service:
-// Mini App launch data (auth) and Login Widget data (linking, Stage 11).
+// Mini App launch data (auth) and Login Widget data (linking).
 // *connector.Client implements it; a nil value disables the telegram auth and
 // telegram-link paths.
 type TelegramValidator interface {
@@ -115,8 +115,8 @@ func NewRegistry(backend *backendclient.Client, tg TelegramValidator, defaultLan
 	r.ops[MsgDraftGet] = Op{Handler: getDraftHandler(backend), Auth: true}
 	r.ops[MsgDraftSave] = Op{Handler: saveDraftHandler(backend), Auth: true}
 	r.ops[MsgGameHide] = Op{Handler: hideGameHandler(backend), Auth: true}
-	registerStage8(r, backend)
-	registerStage11(r, backend, tg, defaultLanguages)
+	registerSocialOps(r, backend)
+	registerLinkOps(r, backend, tg, defaultLanguages)
 	return r
 }
 
@@ -264,7 +264,7 @@ func chatPostHandler(backend *backendclient.Client) Handler {
 	}
 }
 
-// decodeTiles reads the index-addressed tiles to place from a SubmitPlayRequest (Stage 13).
+// decodeTiles reads the index-addressed tiles to place from a SubmitPlayRequest.
 func decodeTiles(in *fb.SubmitPlayRequest) []backendclient.PlayTileJSON {
 	n := in.TilesLength()
 	tiles := make([]backendclient.PlayTileJSON, 0, n)
@@ -282,7 +282,7 @@ func decodeTiles(in *fb.SubmitPlayRequest) []backendclient.PlayTileJSON {
 	return tiles
 }
 
-// decodeEvalTiles reads the index-addressed tentative tiles from an EvalRequest (Stage 13).
+// decodeEvalTiles reads the index-addressed tentative tiles from an EvalRequest.
 func decodeEvalTiles(in *fb.EvalRequest) []backendclient.PlayTileJSON {
 	n := in.TilesLength()
 	tiles := make([]backendclient.PlayTileJSON, 0, n)
@@ -301,7 +301,7 @@ func decodeEvalTiles(in *fb.EvalRequest) []backendclient.PlayTileJSON {
 }
 
 // bytesToInts widens a FlatBuffers ubyte vector (an alphabet-index list) to []int for the
-// backend JSON edge (Stage 13: rack-exchange tiles and the word-check query).
+// backend JSON edge (rack-exchange tiles and the word-check query).
 func bytesToInts(bs []byte) []int {
 	out := make([]int, len(bs))
 	for i, b := range bs {
@@ -429,7 +429,7 @@ func nudgeHandler(backend *backendclient.Client) Handler {
 	}
 }
 
-// getDraftHandler returns the player's saved composition (Stage 17). It reuses
+// getDraftHandler returns the player's saved composition. It reuses
 // GameActionRequest for the game id and wraps the backend's raw JSON in a DraftView.
 func getDraftHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
@@ -442,7 +442,7 @@ func getDraftHandler(backend *backendclient.Client) Handler {
 	}
 }
 
-// saveDraftHandler upserts the player's composition (Stage 17), forwarding the opaque JSON
+// saveDraftHandler upserts the player's composition, forwarding the opaque JSON
 // string verbatim. It echoes an empty DraftView as a well-formed acknowledgement.
 func saveDraftHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
@@ -454,7 +454,7 @@ func saveDraftHandler(backend *backendclient.Client) Handler {
 	}
 }
 
-// hideGameHandler hides a finished game from the caller's own list (Stage 17). It reuses
+// hideGameHandler hides a finished game from the caller's own list. It reuses
 // GameActionRequest for the game id and echoes an Ack.
 func hideGameHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {