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.

docs/UI_DESIGN.md

@@ -5,7 +5,7 @@ Visual and interaction conventions for the `ui` client. Behaviour lives in
 points this doc references) lives in [`ARCHITECTURE.md`](ARCHITECTURE.md). The client
 is **pure HTML5/CSS + Unicode** — no image/font/SVG assets; icons are CSS shapes or
 emoji glyphs. Tokens are CSS custom properties (`ui/src/app.css`), light/dark via
-`prefers-color-scheme` or an explicit Settings choice, and **Telegram-themed** (Stage 9):
+`prefers-color-scheme` or an explicit Settings choice, and **Telegram-themed**:
 on a Telegram Mini App launch — the app is served under `/telegram/` and detects the
 launch by `Telegram.WebApp.initData` — the SDK's `themeParams` override the tokens at
 runtime; opened outside Telegram, the `/telegram/` path redirects to the site root.
@@ -33,14 +33,14 @@ Login uses `Screen`.
   emoji icon over a tiny truncated label. A press highlights a rounded **square** behind
   the icon (slightly larger than it) until release; spacing keeps adjacent labels from
   touching. No text selection on nav / tab-bar / buttons (`user-select: none`).
-- **Screen transitions** (Stage 17, `App.svelte`): navigation slides directionally — a
+- **Screen transitions** (`App.svelte`): navigation slides directionally — a
   screen entered from the lobby flies in from the right; returning to the lobby reveals it
   from the left (back). Transitions are local (so they do not play on first load) and
   collapse to nothing under reduce-motion. Per-game and lobby in-memory caches
   (`lib/gamecache.ts`, `lib/lobbycache.ts`) render a re-opened game or the lobby instantly
   and refresh in the background, removing the blank-loading flash and the lobby's "draw-in"
   on lobby ↔ game navigation.
-- **Telegram integration** (Stage 17, `lib/telegram.ts`): inside the Mini App the colour
+- **Telegram integration** (`lib/telegram.ts`): inside the Mini App the colour
   scheme is forced from `Telegram.WebApp.colorScheme` (over the OS `prefers-color-scheme`,
   which leaks into the Telegram Desktop webview and otherwise fights it) and the Settings
   theme switcher is hidden; the nav bar takes Telegram's background and `setHeaderColor` /
@@ -67,7 +67,7 @@ Login uses `Screen`.
   preventDefault fires only for two touches, so one-finger scroll stays native, and a second
   finger aborts an in-progress drag). The pan is **interpolated toward a pre-clamped target**
   as the real width grows/shrinks, so it magnifies evenly A→B instead of lurching and snapping
-  back near the edges (Stage 17). It **recentres only on a zoom-in** — placing a 2nd+ tile or
+  back near the edges. It **recentres only on a zoom-in** — placing a 2nd+ tile or
   hovering a dragged tile never jumps the board. On touch the first tile placement auto-zooms
   in centred on the target, and **holding a dragged tile over a cell ~1 s** auto-zooms there
   the first time. The swipe-to-open-history gesture stays dropped (it fought native scroll);
@@ -78,15 +78,15 @@ Login uses `Screen`.
   cell is **highlighted as a drop target** (an accent ring). A pending tile is taken back by a
   **double-tap** or by **dragging it back onto the rack** (unzoomed board only — when zoomed
   the one-finger gesture scrolls). A single tap no longer recalls (too easy to trigger); a
-  recalled tile returns to its original rack slot (Stage 17).
-- **Players plaque & history** (`Game.svelte`, Stage 17): the seats above the board share
+  recalled tile returns to its original rack slot.
+- **Players plaque & history** (`Game.svelte`): the seats above the board share
   the width evenly; the seat whose turn it is is **raised** (a drop shadow on its sides)
   while the others read **sunk in** (an inset shadow). A tap anywhere on the plaque toggles
   the **move history** — a fixed-height slide-down drawer whose bottom border (and its
   shadow) pins to the board as the board slides down, instead of tracking the table as
   moves accumulate; its scrollbar gutter is reserved so the centred word column does not
   jitter. A move's row lists every word it formed (the main word first).
-- **Vertical fit & keyboard** (Stage 17): when the game does not fit the viewport, only the
+- **Vertical fit & keyboard**: when the game does not fit the viewport, only the
   board area scrolls vertically (`Screen` `column` mode; the score bar, status, rack and tab
   bar stay fixed), while zoom keeps its own scroll. The check-word dialog opens in
   `Modal` keyboard-overlay mode — the small sheet is top-anchored and the soft keyboard
@@ -99,7 +99,7 @@ Login uses `Screen`.
 - **Bonus-square labels** — a Settings choice (`boardlabels.ts`): `beginner` shows a
   split `3×` / `word` (localized слово/буква), `classic` a single `3W` / `3С`, `none`
   nothing. Default **beginner**.
-- **Grid lines** — a Settings toggle, **default off** (Stage 17). Off: a **gapless
+- **Grid lines** — a Settings toggle, **default off**. Off: a **gapless
   checkerboard** — plain cells alternate two shades, and tiles get rounded corners with a
   soft right-side shadow so adjacent gapless tiles still read apart, reclaiming ~14px of
   board width. On: the classic lined grid, where the inter-cell gap shows a contrasting
@@ -110,7 +110,7 @@ Login uses `Screen`.
 - **HoldConfirm** (`components/HoldConfirm.svelte`): the shared press-and-hold control. A
   short tap opens a small popover above the button; a ~0.7 s hold runs the primary action
   immediately. Used by the **Skip** and **Hint** tabs (each confirmed by an **Ok ✅** popover).
-- **MakeMove / Reset** (Stage 17): when ≥1 tile is pending the rack collapses its used slots
+- **MakeMove / Reset**: when ≥1 tile is pending the rack collapses its used slots
   and shifts left, a **borderless ✅ icon button** (styled like a tab, not a filled accent
   button) beside the rack commits the move — no popover, and disabled while the pending word
   is known illegal; the 🔀 Shuffle tab is replaced by a **↩️ Reset** tab.
@@ -123,7 +123,7 @@ Login uses `Screen`.
 
 ## Announcement banner (`components/AdBanner.svelte`, `lib/banner.ts`)
 
-A one-line inset strip under the nav bar, drawn on a dedicated `--ad-bg` token (Stage 17) —
+A one-line inset strip under the nav bar, drawn on a dedicated `--ad-bg` token —
 a subtle accent, a touch darker than the surroundings in the light theme and a touch lighter
 in the dark theme, mapped to Telegram's `secondary_bg_color` inside the Mini App. Content is
 minimal markdown (text + links, escaped + linkified). A parameterised **rotator** drives messages: a fitting message
@@ -138,7 +138,7 @@ Lobby rows show two lines (opponents, then result + score) with a large place-ba
 on the right: Victory 🏆 / Defeat 🥈 / Draw 🏅, and for 3–4-player games II 🥈 / III 🥉 /
 IV 🏅; active games show Your move 🟢 / Opponent's move ⏳; invitations use 💌.
 
-## Social, account & history surfaces (Stage 8)
+## Social, account & history surfaces
 
 - **Friends** (`screens/Friends.svelte`, from the lobby menu): an "add a friend" block
   pairing a code **input** with a **Show my code** action that reveals a large 6-digit