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/game/Rack.svelte

@@ -20,7 +20,7 @@
     selected: number | null;
     shuffling?: boolean;
     // While a rack tile is being dragged to reorder it, draggingId is its id (hidden here —
-    // the drag ghost stands in) and dropIndex is the slot where a gap opens (Stage 17).
+    // the drag ghost stands in) and dropIndex is the slot where a gap opens.
     draggingId?: number | null;
     dropIndex?: number | null;
     ondown: (e: PointerEvent, index: number) => void;
@@ -93,7 +93,7 @@
     user-select: none;
     -webkit-user-select: none;
     /* iOS shows a tap/active highlight that can linger on the neighbour sliding into a
-       dragged tile's slot (Stage 17); suppress it so only our own styles mark a tile. */
+       dragged tile's slot; suppress it so only our own styles mark a tile. */
     -webkit-tap-highlight-color: transparent;
   }
   .tile.selected {