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.

PLAN.md

@@ -648,7 +648,10 @@ cannot submit; three-way admin filter.
   - **Margin** (interview): pick the candidate whose resulting margin (own+move−opp)
     is closest to **[1,30]** when playing to win / **[−30,−1]** when playing to lose,
     tie-broken toward the conservative edge; no legal play → exchange the full rack
-    when the bag can refill it, else pass.
+    when the bag can refill it, else pass. *(Later refined — separate PR: on ≈20% of
+    opening/midgame moves the robot flips its intent for that one move, the chance
+    tapering to 0 over the last 14 bag tiles and 0 at an empty bag, so the endgame
+    stays strict; deterministic from the seed.)*
   - **Substitution** (interview): a matchmaker **reaper** (`Reap`/`RunReaper`)
     substitutes a pooled robot after a **10 s** wait (`BACKEND_LOBBY_ROBOT_WAIT`),
     `NewMatchmaker` now takes a `RobotProvider`. A waiter learns of a match — human