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.

ui/src/lib/codec.ts

@@ -38,7 +38,7 @@ import type {
 
 // --- request encoders ---
 
-// buildPlayTile encodes one to-place tile by its alphabet index (Stage 13); a placed blank
+// buildPlayTile encodes one to-place tile by its alphabet index; a placed blank
 // carries its designated letter's index with blank set.
 function buildPlayTile(b: Builder, t: PlacedTile, variant: Variant): Offset {
   fb.PlayTile.startPlayTile(b);
@@ -73,7 +73,7 @@ export function encodeStateRequest(gameId: string, includeAlphabet: boolean): Ui
   return finish(b, fb.StateRequest.endStateRequest(b));
 }
 
-// encodeDraftSave wraps the player's composition JSON (Stage 17). The string is opaque on the
+// encodeDraftSave wraps the player's composition JSON. The string is opaque on the
 // wire — the gateway forwards it verbatim and only the client reads {rack_order, board_tiles}.
 export function encodeDraftSave(gameId: string, json: string): Uint8Array {
   const b = new Builder(256);
@@ -324,7 +324,7 @@ export function decodeProfile(buf: Uint8Array): Profile {
 
 // decodeStateViewTable projects a StateView table (a root or one nested in an event) to the
 // model. It caches the alphabet when present (a per-variant cache miss) and decodes the index
-// rack to display letters with it (Stage 13).
+// rack to display letters with it.
 function decodeStateViewTable(v: fb.StateView): StateView {
   const g = v.game();
   const variant = (g ? s(g.variant()) : 'scrabble_en') as Variant;
@@ -355,7 +355,7 @@ export function decodeMoveResult(buf: Uint8Array): MoveResult {
   const r = fb.MoveResult.getRootAsMoveResult(new ByteBuffer(buf));
   const m = r.move();
   const g = r.game();
-  // The actor's refilled rack rides back as alphabet indices (R4); decode it with the game's variant.
+  // The actor's refilled rack rides back as alphabet indices; decode it with the game's variant.
   const variant = (g ? s(g.variant()) : 'scrabble_en') as Variant;
   const rack: string[] = [];
   for (let i = 0; i < r.rackLength(); i++) rack.push(letterForIndex(variant, r.rack(i) ?? 0));
@@ -493,7 +493,7 @@ export function decodeEvent(kind: string, payload: Uint8Array): PushEvent | null
   }
 }
 
-// --- Stage 8 encoders ---
+// --- social encoders ---
 
 export function encodeTarget(accountId: string): Uint8Array {
   const b = new Builder(64);
@@ -563,7 +563,7 @@ export function encodeUpdateProfile(p: ProfileUpdate): Uint8Array {
   return finish(b, fb.UpdateProfileRequest.endUpdateProfileRequest(b));
 }
 
-// --- account linking & merge (Stage 11) ---
+// --- account linking & merge ---
 
 export function encodeLinkEmailRequest(email: string): Uint8Array {
   const b = new Builder(128);
@@ -604,7 +604,7 @@ export function decodeLinkResult(buf: Uint8Array): LinkResult {
   };
 }
 
-// --- Stage 8 decoders ---
+// --- social decoders ---
 
 function decodeAccountRef(r: fb.AccountRef): AccountRef {
   return { accountId: s(r.accountId()), displayName: s(r.displayName()) };