feat(offline): local game engine (Phase B1)

The offline vs_ai game engine — a faithful TS port of backend/internal/engine that
drives a whole local game with no backend. Composes with the move generator (#188) and
robot strategy (#189) from Phase A; not yet wired into the UI (Phase B2/B3).

- ui/src/lib/localgame/ruleset.ts: static per-variant tile values / bag counts / blank
  count, mirrored from rules.go (offline scoring is self-contained; online uses the
  server alphabet). Pinned by ruleset.parity.test.ts against a Go fixture.
- bag.ts: the tile bag (fill from counts/blanks, draw-from-end, return+reshuffle) on a
  deterministic in-house PRNG — a game replays from its seed, not bit-identical to a
  server game (per plan).
- board.ts: the mutable board, satisfying the validator/generator read view + set().
- engine.ts: LocalGame — deal / play (reusing validate.ts) / pass / exchange / resign,
  scoreless(6) & out-of-tiles end detection, end-of-game rack penalties, winner; mirrors
  game.go. The end-game math is exported as pure functions, pinned against the Go engine
  (engine.parity.test.ts, 9 constructed positions).
- engine.test.ts: a full-loop smoke — two robots play a whole vs_ai game to completion
  via generateMoves + decide, and it is reproducible from the seed.
- backend: movegen now dumps the per-variant rulesets; a new in-package engine emitter
  (endfixture_test.go, env-gated) produces the end-game golden.

Pure additive library code; no runtime behavior change (unused at runtime, bundle unchanged).

ui/src/lib/localgame/testdata/endgame.json

@@ -0,0 +1,246 @@
+{
+  "cases": [
+    {
+      "name": "out-basic",
+      "variant": "scrabble_en",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        [
+          0,
+          1,
+          2
+        ]
+      ],
+      "scores": [
+        50,
+        40
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        57,
+        33
+      ],
+      "winner": 0
+    },
+    {
+      "name": "out-blank",
+      "variant": "scrabble_en",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        [
+          255,
+          0
+        ]
+      ],
+      "scores": [
+        30,
+        30
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        31,
+        29
+      ],
+      "winner": 0
+    },
+    {
+      "name": "out-erudit-yo",
+      "variant": "erudit_ru",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        [
+          6,
+          32
+        ]
+      ],
+      "scores": [
+        10,
+        10
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        13,
+        7
+      ],
+      "winner": 0
+    },
+    {
+      "name": "out-tie",
+      "variant": "scrabble_en",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        []
+      ],
+      "scores": [
+        30,
+        30
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        30,
+        30
+      ],
+      "winner": -1
+    },
+    {
+      "name": "out-3p",
+      "variant": "scrabble_ru",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        [
+          0,
+          1
+        ],
+        [
+          2
+        ]
+      ],
+      "scores": [
+        10,
+        10,
+        10
+      ],
+      "resigned": [
+        false,
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        15,
+        6,
+        9
+      ],
+      "winner": 0
+    },
+    {
+      "name": "scoreless",
+      "variant": "scrabble_en",
+      "reason": "scoreless",
+      "hands": [
+        [
+          0,
+          1
+        ],
+        [
+          2,
+          3
+        ]
+      ],
+      "scores": [
+        20,
+        20
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        16,
+        15
+      ],
+      "winner": 0
+    },
+    {
+      "name": "resign-2p",
+      "variant": "scrabble_en",
+      "reason": "resign",
+      "hands": [
+        [],
+        [
+          0
+        ]
+      ],
+      "scores": [
+        100,
+        10
+      ],
+      "resigned": [
+        true,
+        false
+      ],
+      "toMove": 1,
+      "scoresAfter": [
+        100,
+        10
+      ],
+      "winner": 1
+    },
+    {
+      "name": "resign-3p",
+      "variant": "scrabble_en",
+      "reason": "resign",
+      "hands": [
+        [],
+        [],
+        []
+      ],
+      "scores": [
+        50,
+        60,
+        40
+      ],
+      "resigned": [
+        false,
+        true,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        50,
+        60,
+        40
+      ],
+      "winner": 0
+    },
+    {
+      "name": "aborted",
+      "variant": "scrabble_en",
+      "reason": "aborted",
+      "hands": [
+        [
+          0
+        ],
+        [
+          1
+        ]
+      ],
+      "scores": [
+        40,
+        30
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        40,
+        30
+      ],
+      "winner": -1
+    }
+  ]
+}