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).

.gitea/workflows/go-unit.yaml

@@ -31,6 +31,13 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Fetch scrabble-solver (sibling)
+        # The engine package consumes scrabble-solver in-process; go.work points
+        # its bare module path at this sibling checkout. The repository is public,
+        # so the clone needs no credentials. It tracks master HEAD (see PLAN.md
+        # TODO-1 for the move to a published, versioned module).
+        run: git clone --depth 1 https://gitea.iliadenisov.ru/developer/scrabble-solver.git "${GITHUB_WORKSPACE}/../scrabble-solver"
+
       - name: Set up Go
         uses: actions/setup-go@v5
         with:
@@ -52,5 +59,8 @@ jobs:
 
       - name: test
         # -count=1 disables the test cache so a green run never depends on a
-        # previous runner's cached state.
+        # previous runner's cached state. BACKEND_DICT_DIR points the engine
+        # tests at the committed DAWGs in the sibling checkout.
+        env:
+          BACKEND_DICT_DIR: ${{ github.workspace }}/../scrabble-solver/dawg
         run: go test -count=1 ./backend/...