Stage 5: robot opponent (pool, seed-derived strategy, move driver, matchmaker substitution)

- internal/robot: durable kind='robot' account pool (migration 00004); every
  per-game and per-turn choice derived deterministically from the game seed
  (restart-stable FNV mix); a background move driver; margin targeting (band
  1-30, closest-to-band); right-skewed [2,90]min delays (median ~10m);
  opponent-anchored sleep with +/-3h drift; daytime nudge reply + proactive
  12h nudge; friend/chat blocked via profile toggles.
- engine.Candidates (decoded ranked plays); game.Candidates + RobotTurns;
  social.LastNudgeAt.
- matchmaker: 10s wait then robot substitution (reaper) + Poll delivery seam.
- config (BACKEND_ROBOT_DRIVE_INTERVAL, BACKEND_LOBBY_ROBOT_WAIT,
  BACKEND_LOBBY_REAPER_INTERVAL); main wiring + boot-time pool provisioning.
- metrics: robot account_stats (authoritative balance) + robot_games_finished_total
  OTel counter + per-finish log.
- docs: PLAN, ARCHITECTURE, FUNCTIONAL(+ru), TESTING, README; account.go comment.
- tests: robot strategy units, matchmaker reaper/Poll, engine.Candidates; inttest
  robot full-game / substitution / proactive-nudge.

docs/TESTING.md

@@ -32,21 +32,30 @@ tests or touching CI.
   Postgres-backed integration tests in `inttest` (full lifecycle to a natural
   end, **journal-replay equivalence**, the turn-timeout sweep with away-window
   grace, resign win/loss and statistics, the hint allowance-then-wallet policy,
-  word-check and complaint capture, and per-game-lock serialisation). The robot
-  balance/margin regression tests arrive with Stage 5. Stage 4 adds the engine's
-  **multi-player drop-out** cases (continue after one resign, last-survivor win,
-  the tile-disposition bag effect) and a domain integration test for a 3-player
-  **timeout that continues**.
+  word-check and complaint capture, and per-game-lock serialisation). Stage 4 adds
+  the engine's **multi-player drop-out** cases (continue after one resign,
+  last-survivor win, the tile-disposition bag effect) and a domain integration test
+  for a 3-player **timeout that continues**. The engine also gains a `Candidates`
+  ranked/decoded test (Stage 5).
 - **Social & lobby** *(Stage 4+)* — `backend/internal/social` unit-tests the chat
   **content filter** (links/emails/phones plus obfuscated forms) and
   `backend/internal/lobby` unit-tests the in-memory **matchmaker** (FIFO pairing,
-  cancel, per-variant pools) with a fake game creator. Postgres-backed `inttest`
-  covers the friend request/accept lifecycle with the block/toggle guards, the
-  per-user block (and its severing of friendships), chat post/list with the IP,
+  cancel, per-variant pools, plus the Stage 5 **robot substitution** reaper and
+  `Poll` delivery) with fake game-creator and robot-provider seams. Postgres-backed
+  `inttest` covers the friend request/accept lifecycle with the block/toggle guards,
+  the per-user block (and its severing of friendships), chat post/list with the IP,
   content and block-visibility rules, the nudge turn/rate-limit rules, the
   invitation flow (all-accept starts the game, decline cancels, lazy expiry,
   inviter-only cancel), and the email confirm-code flow (request/confirm, taken
   email, expiry and attempt-cap) with a fixture mailer.
+- **Robot** *(Stage 5+)* — `backend/internal/robot` unit-tests the pure strategy:
+  the ≈ 40% play-to-win split over many seeds, the right-skewed move-delay
+  (bounds, ~10-min median, determinism), the margin selection (win/lose, in-band
+  and out-of-band fallbacks, no-play exchange/pass), the sleep window with drift
+  and the midnight wrap, and mix restart-stability. Postgres-backed `inttest`
+  drives a robot through a full auto-match to a natural end (asserting a robot
+  statistics row), the matchmaker substitution end-to-end (enqueue → reap →
+  `[human, robot]`, discoverable via `Poll`), and a proactive 12-hour nudge.
 
 ## Principles