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).

go.work.sum

@@ -18,6 +18,10 @@ github.com/golang-sql/civil v0.0.0-20220223132316-b832511892a9/go.mod h1:8vg3r2V
 github.com/golang-sql/sqlexp v0.1.0/go.mod h1:J4ad9Vo8ZCWQ2GMrC4UCQy1JpCbwU9m3EOqtpKwwwHI=
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+github.com/iliadenisov/alphabet v1.1.0 h1:d87N7Rmpjj9FgL7bvEaqLdaIaNch2hC6HvkbKGhn7Hk=
+github.com/iliadenisov/alphabet v1.1.0/go.mod h1:h6BhDBiJBLhMEb5XfsqJXZop3hhwXaD8lc5yf38Baqw=
+github.com/iliadenisov/dafsa v1.1.0 h1:NV1ZOstMdHXI/cCyAZKOD3qnKLoYdMUunA0+Baj7vR4=
+github.com/iliadenisov/dafsa v1.1.0/go.mod h1:mG6Y0DdfRrqdXGqTEMb9Zx0Fl0NkP3ZDYesvxR+e14o=
 github.com/jackc/chunkreader v1.0.0 h1:4s39bBR8ByfqH+DKm8rQA3E1LHZWB9XWcrz8fqaZbe0=
 github.com/jackc/pgproto3 v1.1.0 h1:FYYE4yRw+AgI8wXIinMlNjBbp/UitDJwfj5LqqewP1A=
 github.com/jackc/puddle v1.3.0 h1:eHK/5clGOatcjX3oWGBO/MpxpbHzSwud5EWTSCI+MX0=
@@ -54,6 +58,7 @@ github.com/ydb-platform/ydb-go-sdk/v3 v3.135.0/go.mod h1:VYUUkRJkKuQPkIpgtZJj6+5
 github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78/go.mod h1:aL8wCCfTfSfmXjznFBSZNN13rSJjlIOI1fUNAtF7rmI=
 github.com/ziutek/mymysql v1.5.4/go.mod h1:LMSpPZ6DbqWFxNCHW77HeMg9I646SAhApZ/wKdgO/C0=
 go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
+golang.org/x/exp v0.0.0-20260410095643-746e56fc9e2f h1:W3F4c+6OLc6H2lb//N1q4WpJkhzJCK5J6kUi1NTVXfM=
 golang.org/x/exp v0.0.0-20260410095643-746e56fc9e2f/go.mod h1:J1xhfL/vlindoeF/aINzNzt2Bket5bjo9sdOYzOsU80=
 golang.org/x/mod v0.32.0/go.mod h1:SgipZ/3h2Ci89DlEtEXWUk/HteuRin+HHhN+WbNhguU=
 golang.org/x/mod v0.34.0/go.mod h1:ykgH52iCZe79kzLLMhyCUzhMci+nQj+0XkbXpNYtVjY=