feat(variants): default a registered account to Erudit + Russian Scrabble

New Game opened on a single variant for everyone, which reads poorly on VK
where Russian Scrabble is the familiar game. A registered account now starts
with both Russian-alphabet games, so the picker offers a real choice with no
pre-selection.

A guest stays on Erudit alone and is invited to register for the rest: the
device-local guest has no profile at all, so the client-side fallback has to
keep matching the server. Registering promotes the set, but only while the
guest still carries the untouched guest default. Existing accounts are not
backfilled — the set they carry may be a deliberate choice.

The Telegram promo start-param becomes additive rather than a replacement,
with English Scrabble withheld from a Russian-speaking arrival; otherwise the
English campaign link would have taken Russian Scrabble away from every
account it onboarded.

docs/ARCHITECTURE.md

@@ -245,9 +245,15 @@ arrive from a platform rather than completing a mandatory registration).
   than stranding a user who never opened Settings on the creation-time seed.
 - **Variant preferences (New Game gating).** Which variants a player may be matched into is a
   per-user **profile** setting — `variant_preferences`, a set of `engine.Variant` labels
-  (`scrabble_en`, `scrabble_ru`, `erudit_ru`) edited on the Settings/Profile screen. New
-  accounts default to **Erudit only** (a DB column default); at least one variant must stay
-  selected. The picker is ordered **Erudit-first** everywhere. The preference gates the New
+  (`scrabble_en`, `scrabble_ru`, `erudit_ru`) edited on the Settings/Profile screen. A newly
+  **registered** account defaults to the two Russian-alphabet games — **Erudit + Russian
+  Scrabble** (a DB column default); a **guest** is deliberately narrower and starts on
+  **Erudit only** (`account.ProvisionGuest` writes the set explicitly), with New Game inviting
+  the guest to register for the rest. Registering promotes the set to the registered default
+  (`account.ClearGuest`), but only while the guest still carries the untouched guest set —
+  a player who chose their own set is never overruled, and **existing accounts are not
+  backfilled** when the default moves. At least one variant must stay selected. The picker is
+  ordered **Erudit-first** everywhere. The preference gates the New
   Game picker on every create path the player **initiates** — auto-match, vs-AI and a friend
   invitation the player **creates** — and the backend **enforces** it on those paths (a chosen
   variant outside the caller's preferences is rejected with HTTP 400). An **invited** friend
@@ -316,6 +322,18 @@ arrive from a platform rather than completing a mandatory registration).
 > `service_language`/`supported_languages` and the push `language` routing field, and
 > gained `variant_preferences` on Profile/UpdateProfile.
 
+> **Decision (2026-07-27) — registered accounts start on both Russian games.** The
+> Erudit-only default was too narrow a first impression on VK, where Russian Scrabble is the
+> familiar game: a **registered** account is now created with `erudit_ru + scrabble_ru`, so
+> New Game opens with a real choice (and therefore no pre-selected variant). A **guest**
+> stays on Erudit alone and is invited to register for the rest, which keeps the offline
+> device-local guest — who has no profile at all — matching the server. **Existing accounts
+> are not backfilled**: the set a player already carries may be a deliberate choice, and the
+> two are indistinguishable in the data. The Telegram promo `start_param` correspondingly
+> became **additive** rather than a replacement (it can only widen a set), with English
+> Scrabble withheld from a Russian-speaking arrival — otherwise the English campaign link
+> would have quietly taken Russian Scrabble away from every account it onboarded.
+
 ## 4. Accounts, identities, linking & merge
 
 - One internal account may carry several **platform identities**
@@ -883,7 +901,8 @@ in either direction (the enqueue excludes the caller's `BlockedWith` set);
 - **Profile**: `preferred_language` (en/ru; tracks the interface language — §4), display name, email
   (confirm-code binding, see §4), **timezone**, the daily **away window**, the
   **variant preferences** (`variant_preferences`, the matchable-variant set that gates New
-  Game — §3, defaulting to Erudit only, at least one enforced) and the
+  Game — §3, defaulting to Erudit + Russian Scrabble for a registered account and Erudit
+  alone for a guest, at least one enforced) and the
   block toggles — all editable through `account.UpdateProfile`, which validates them:
   a display name is Unicode letters joined by single ` `/`.`/`_`
   separators (no leading/trailing/adjacent separators, ≤ 32 runes); the timezone is a