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/premiums.ts

@@ -2,7 +2,7 @@
 // of truth, scrabble-solver/rules/rules.go (standardBoard / eruditBoard). The board is not
 // transmitted on the wire (StateView has no board), so the client renders the premiums
 // locally; only the centre differs by variant. A Vitest parity test pins the geometry.
-// Tile values and the alphabet moved to the server-sent per-variant table in Stage 13 (see
+// Tile values and the alphabet come from the server-sent per-variant table (see
 // lib/alphabet.ts), so this file is geometry only.
 
 import type { Variant } from './model';
@@ -85,5 +85,5 @@ export function centre(variant: Variant): { row: number; col: number } {
   return { row: 7, col: 7 };
 }
 
-// Tile values and the per-variant alphabet now arrive from the server (lib/alphabet.ts,
-// Stage 13); the board geometry above is all this module owns.
+// Tile values and the per-variant alphabet now arrive from the server (lib/alphabet.ts);
+// the board geometry above is all this module owns.