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.

loadtest/internal/moves/moves.go

@@ -18,11 +18,11 @@ import (
 	"scrabble/loadtest/internal/edge"
 )
 
-// blankIndex is the rack/exchange sentinel for a blank tile on the wire (Stage 13).
+// blankIndex is the rack/exchange sentinel for a blank tile on the wire.
 const blankIndex = 255
 
 // variantSpec maps an edge variant label to its ruleset constructor and committed
-// DAWG filename (the descriptive names kept by R1).
+// DAWG filename (using descriptive names).
 type variantSpec struct {
 	ruleset func() *rules.Ruleset
 	dawg    string