feat(offline): port DAWG cursor + move generator to TS (parity-pinned)

First engine-first step of PWA offline mode (Phase A): the client-side move
generator — the "robot brain" a local vs_ai game will run on-device — with no
runtime wiring yet (Phase B).

- dawg.ts: add the step-by-step cursor (root/final/next/arcs), a faithful port
  of dafsa traverse.go over the reader's existing bitstream.
- generate.ts: the Appel-Jacobson generator (leftPart/extendRight + cross-sets +
  counts-rack + board transpose + moveKey ranking), reusing the cursor and
  validate.ts evaluate/connected. A cross-set LetterSet is a Uint8Array, so the
  33-letter Russian alphabet (index 32) is exact under JS bit ops.
- validate.ts: export connected for the generator's connectivity filter.
- backend/cmd/movegen: dev tool building small sample dictionaries and emitting
  golden move-generation fixtures from the real Go solver (EN + RU).
- tests: dawg.cursor.test.ts (enumeration bijection vs indexOf) and
  generate.parity.test.ts (7/7 vs the Go solver: empty board, mid-game, blank,
  single-word rule, Russian index-32 cross-set). The committed EN sample also
  unblocks the existing skipped dawg.parity.test.ts once wired with DICT_* in CI.

Pure additive library code; no runtime behavior change.

ui/src/lib/dict/testdata/sample_en.words.json

@@ -0,0 +1,122 @@
+{
+  "alphabet": "en",
+  "numAdded": 18,
+  "words": [
+    "a",
+    "an",
+    "and",
+    "ant",
+    "car",
+    "care",
+    "cared",
+    "cares",
+    "cars",
+    "cat",
+    "cats",
+    "do",
+    "doe",
+    "does",
+    "dog",
+    "dogs",
+    "done",
+    "dot"
+  ],
+  "indexes": [
+    [
+      0
+    ],
+    [
+      0,
+      13
+    ],
+    [
+      0,
+      13,
+      3
+    ],
+    [
+      0,
+      13,
+      19
+    ],
+    [
+      2,
+      0,
+      17
+    ],
+    [
+      2,
+      0,
+      17,
+      4
+    ],
+    [
+      2,
+      0,
+      17,
+      4,
+      3
+    ],
+    [
+      2,
+      0,
+      17,
+      4,
+      18
+    ],
+    [
+      2,
+      0,
+      17,
+      18
+    ],
+    [
+      2,
+      0,
+      19
+    ],
+    [
+      2,
+      0,
+      19,
+      18
+    ],
+    [
+      3,
+      14
+    ],
+    [
+      3,
+      14,
+      4
+    ],
+    [
+      3,
+      14,
+      4,
+      18
+    ],
+    [
+      3,
+      14,
+      6
+    ],
+    [
+      3,
+      14,
+      6,
+      18
+    ],
+    [
+      3,
+      14,
+      13,
+      4
+    ],
+    [
+      3,
+      14,
+      19
+    ]
+  ]
+}

ui/src/lib/dict/testdata/sample_ru.words.json

@@ -0,0 +1,38 @@
+{
+  "alphabet": "ru",
+  "numAdded": 6,
+  "words": [
+    "ад",
+    "ар",
+    "оса",
+    "я",
+    "яд",
+    "яр"
+  ],
+  "indexes": [
+    [
+      0,
+      4
+    ],
+    [
+      0,
+      17
+    ],
+    [
+      15,
+      18,
+      0
+    ],
+    [
+      32
+    ],
+    [
+      32,
+      4
+    ],
+    [
+      32,
+      17
+    ]
+  ]
+}