Stage 17: fix the robot-nudge frequency + per-game push language

Two owner-reported defects from a live contour game.

A. Frequency: the robot's proactive nudge fired hourly for 12h+ (a 12h idle threshold
   then the 1h cooldown, uncapped). Replaced with a lengthening, randomized schedule
   (proactiveNudgeGap): the first nudge ~60-90 min into the human's turn, each later gap
   growing toward 1-6h (uniform sample in [60min, ceil], ceil ramping 90min->6h over 12h
   of idle, measured from the previous nudge), so a long wait gets a handful of
   increasingly-spaced reminders instead of a stream.

B. Language: out-of-app push routed by the recipient's GLOBAL service_language
   (last-login-wins), so after re-logging via the RU bot an English game's nudges came
   from the RU bot. Now a game push (your_turn, game_over, nudge, match_found) carries
   the game's own language (engine.Variant.Language) on push.Event, and the gateway
   routes by it (falling back to service_language for non-game pushes). The New-Game
   variant-gating guarantees the game's bot is one the player has started, so delivery is
   never blocked.

Tests: proactiveNudgeGap unit + retimed TestRobotProactiveNudge; TestVariantLanguage;
emit your_turn/game_over language; TestNudgeRoutedByGameLanguage integration. Docs:
ARCHITECTURE (§7 nudge, §10/§13 routing), FUNCTIONAL (+ _ru), PLAN tracker.

docs/FUNCTIONAL.md

@@ -35,8 +35,10 @@ the Telegram colours, and — on first contact — seeds the new account's inter
 language from the Telegram client. The sign-in service also declares the **game
 languages** it offers (a set of en/ru, at least one), which gate the New Game variant
 choice in the lobby. Telegram runs a separate bot per language (an English bot and a
-Russian bot, the same player spanning both); the bot a player signed in through both
-sets their offered languages and is the bot their out-of-app notifications come from. Guests are session-only with restricted features
+Russian bot, the same player spanning both); the bot a player signed in through sets their
+offered languages, and their non-game notifications come from it. A **game's** notifications
+(your turn, game over, a nudge), though, always come from **that game's** bot — by the game's
+language, not whichever bot the player signed in through last. Guests are session-only with restricted features
 (auto-match only; no friends, stats or history); an abandoned guest that never
 joined a game and has been idle past the retention window is garbage-collected. While the app is open the client
 keeps a live stream and receives in-app updates in real time — the opponent's move,