Stage 2: engine package over scrabble-solver (registry, bag, Game, replay)

backend/internal/engine wraps the sibling scrabble-solver library in-process:

- Registry: versioned DAWG load via dafsa.Load, keyed by (variant, dict_version),
  latest-per-variant; English / Russian / Эрудит handled uniformly.
- Bag: own deterministic, seeded tile bag with Draw + Return (for exchanges),
  since the solver's self-play bag cannot return tiles.
- Game: pure rules engine — deal, play/pass/exchange/resign, refill, per-move
  scoring, turn order, and end-condition detection (empty bag + empty rack, six
  scoreless turns, resignation) with end-game rack adjustment.
- decode/ReplayBoard: dictionary-independent MoveRecords and board replay via
  scrabble.Apply (no internal/encoding), realising ARCHITECTURE §9.1.

Wiring: go.work gains "replace scrabble-solver => ../scrabble-solver"; backend
requires scrabble-solver (placeholder) and github.com/iliadenisov/dafsa directly.
Both Go CI workflows clone the public solver sibling (master HEAD, no token) and
set BACKEND_DICT_DIR.

Docs: ARCHITECTURE §5/§14, TESTING engine layer, backend README, and PLAN
refinements + deferred TODOs (publish/version solver; split engine vs dictionary
generator).

backend/go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/gin-gonic/gin v1.12.0
 	github.com/go-jet/jet/v2 v2.14.1
 	github.com/google/uuid v1.6.0
+	github.com/iliadenisov/dafsa v1.1.0
 	github.com/jackc/pgx/v5 v5.9.2
 	github.com/pressly/goose/v3 v3.27.1
 	github.com/testcontainers/testcontainers-go v0.42.0
@@ -19,6 +20,7 @@ require (
 	go.opentelemetry.io/otel/sdk/metric v1.43.0
 	go.opentelemetry.io/otel/trace v1.43.0
 	go.uber.org/zap v1.27.1
+	scrabble-solver v0.0.0-00010101000000-000000000000
 )
 
 require (