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/e2e/game.spec.ts

@@ -16,7 +16,7 @@ async function openGame(page: Page): Promise<void> {
   await expect(page.locator('.pane')).toHaveCount(1);
 }
 
-test('offline shows the Connecting indicator and softly disables server actions (Stage 17)', async ({ page }) => {
+test('offline shows the Connecting indicator and softly disables server actions', async ({ page }) => {
   await openGame(page);
   // The exchange/draw tab is a server action; on my turn with tiles in the bag it is live.
   const draw = page.locator('.tab').first();
@@ -56,7 +56,7 @@ test('a placed tile is saved as a draft and restored on reopening the game', asy
   await page.waitForTimeout(600); // let the debounced draft save flush to the mock store
 
   // Leave the game and reopen it. The mock keeps the saved composition, so the pending tile is
-  // restored without re-placing it (Stage 17 #4/#6).
+  // restored without re-placing it.
   await page.evaluate(() => (location.hash = '/'));
   await page.getByRole('button', { name: /Ann/ }).click();
   await expect(page.locator('[data-cell]').first()).toBeVisible();
@@ -77,7 +77,7 @@ test('a pending tile recalls on double-tap, not on a single tap', async ({ page
   await page.locator('[data-cell]:not(.filled)').nth(30).click();
   await expect(page.locator('[data-cell].pending')).toHaveCount(1);
 
-  // A single tap must NOT recall it (changed in Stage 17 — recall was too easy to trigger).
+  // A single tap must NOT recall it (recall was too easy to trigger).
   await page.waitForTimeout(350); // clear the double-tap window from the placing tap
   await page.locator('[data-cell].pending').first().click();
   await expect(page.locator('[data-cell].pending')).toHaveCount(1);
@@ -158,7 +158,7 @@ test('dropping the game ends it and shows the result', async ({ page }) => {
   await expect(page.locator('.status .over')).toBeVisible();
 });
 
-test('a placed tile drags from one board cell to another (Stage 17 relocation)', async ({ page }) => {
+test('a placed tile drags from one board cell to another (relocation)', async ({ page }) => {
   await openGame(page);
   await page.locator('.rack .tile').first().click();
   await page.locator('[data-cell]:not(.filled)').nth(30).click();
@@ -181,7 +181,7 @@ test('a placed tile drags from one board cell to another (Stage 17 relocation)',
   expect(to).not.toBe(from);
 });
 
-test('chat and word-check open as their own screens and back to the game (Stage 17)', async ({ page }) => {
+test('chat and word-check open as their own screens and back to the game', async ({ page }) => {
   await openGame(page);
 
   await page.locator('.burger').click();

ui/e2e/landing.spec.ts

@@ -1,7 +1,7 @@
 import { expect, test } from './fixtures';
 
 // The landing page is a separate Vite entry (landing.html), served at "/" in production while
-// the game SPA lives at /app/ and /telegram/ (Stage 17). In dev it is reachable at /landing.html.
+// the game SPA lives at /app/ and /telegram/. In dev it is reachable at /landing.html.
 test('landing shows the pitch, switches language via the dropdown, and toggles theme', async ({ page }) => {
   await page.goto('/landing.html');
 

ui/e2e/social.spec.ts

@@ -1,6 +1,6 @@
 import { expect, test, type Page } from './fixtures';
 
-// Stage 8 social / account / history surfaces against the mock transport (no backend).
+// Social / account / history surfaces against the mock transport (no backend).
 // The mock profile is a durable account, so friends, invitations, stats and the GCG
 // export are reachable from the seeded fixture.
 
@@ -154,7 +154,7 @@ test('game: an opponent who is already a friend shows a disabled "in friends"',
   await loginLobby(page);
   await page.getByRole('button', { name: /Kaya/ }).click(); // the finished game vs Kaya, a seeded friend
   await page.locator('.burger').first().click();
-  // The in-game friend item is derived from the server's friend list (Stage 17): a friend reads
+  // The in-game friend item is derived from the server's friend list: a friend reads
   // a disabled "✓ in friends", not the addable "Add to friends".
   const inFriends = page.getByRole('button', { name: /in friends/i });
   await expect(inFriends).toBeVisible();
@@ -210,7 +210,7 @@ test('chat: the message field shows on your turn, the nudge replaces it otherwis
   await page.getByRole('button', { name: /Ann/ }).click(); // g1: your turn
   await page.locator('.burger').first().click();
   await page.getByRole('button', { name: 'Chat' }).click();
-  // On your turn the message field + Send are shown and the nudge is hidden (Stage 17);
+  // On your turn the message field + Send are shown and the nudge is hidden;
   // chat and nudge are mutually exclusive by turn. Icon-only controls expose their action
   // through the aria-label.
   await expect(page.getByRole('button', { name: 'Send' })).toBeVisible();