feat(robot): occasional off-strategy deviation, strict in the endgame

The robot followed its per-game playToWin/lose intent on every move, which made
the outcome too predictable. It now flips that intent for a single move on ~20%
of opening/midgame turns (a winning robot eases off, a losing one surges ahead),
so the chosen strategy may not pan out — which favours the human. The chance
tapers linearly to 0 over the last 14 tiles in the bag and is 0 once the bag is
empty, so the endgame follows the chosen strategy strictly.

The decision is deterministic from the seed (mix(seed,"deviate",moveCount)) and
applies to both robot paths via the shared selectMove; the per-game play-to-win
intent the admin card shows is unchanged. Adds deviateProb/deviates helpers and
unit tests (taper bounds + monotonicity, never-in-endgame, determinism, ~20%
distribution); bakes the behaviour into ARCHITECTURE §7, FUNCTIONAL (+_ru),
backend/README, PRERELEASE and PLAN Stage 5.

backend/README.md

@@ -50,7 +50,8 @@ each a `kind='robot'` identity, provisioned at startup with chat and friend
 requests blocked — backs human-like, per-language composed names. A background driver plays the
 robot's moves through the public game API as an ordinary seated player (so only
 `internal/engine` imports the solver): it decides once per game whether to play to
-win (≈ 40%), targets a small score margin, and times its moves with a move-number-aware
+win (≈ 40%), targets a small score margin — with an occasional off-strategy move that tapers to
+none as the bag empties — and times its moves with a move-number-aware
 right-skewed delay (quick openings, long endgames), a night-sleep window anchored to the opponent's timezone, and nudge
 behaviour — all derived deterministically from the game seed, so it keeps no extra
 state. A background **reaper** seats a pooled robot (matching the game's language) in any open