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/Board.svelte

@@ -94,7 +94,7 @@
     return () => cancelAnimationFrame(raf);
   });
 
-  // Pinch zoom (Stage 17): a two-finger spread zooms in toward the pinch midpoint, a pinch
+  // Pinch zoom: a two-finger spread zooms in toward the pinch midpoint, a pinch
   // close zooms out. preventDefault fires only for two touches, so the one-finger native
   // scroll of the zoomed board is left untouched. It maps to the same two-state zoom as
   // double-tap, toggling toward the midpoint cell.
@@ -278,7 +278,7 @@
   .cell.pending {
     background: var(--tile-pending);
     /* The placed tile owns the pointer so it can be dragged to relocate it (even on the zoomed
-       board) instead of the touch starting a board pan (Stage 17). */
+       board) instead of the touch starting a board pan. */
     touch-action: none;
   }
   /* Lines-off variant: a gapless checkerboard. The 1px grid gaps (and the cell-line they