P1.1: pure-sx Gem & Board model with seeded, match-free init

Add board.sx, the headless Phase-1 match-3 core:
- Gem enum (6 types, ordinal 0..5) + single-char dump alphabet.
- Rng: a 32-bit LCG carried in s64, masked to 32 bits each step, so the
  stream is host-width independent and valid for any seed.
- Board (8x8, row-major) with idx/at/set accessors and a seeded init that
  fills row-major, excluding any gem that would complete a 3-in-a-row with
  the two cells to the left or above — so the result has zero pre-existing
  matches. Single RNG draw per cell, always terminates.
- board_dump: deterministic one-row-per-line textual snapshot.

tests/board_init.sx seeds with a fixed seed, dumps the board, and asserts
zero horizontal/vertical 3-in-a-row runs via an independent scan. Output and
exit code are locked as goldens. App ios-sim build is unaffected (main.sx
does not import the model yet).

tests/board_init.sx

@@ -0,0 +1,39 @@
+// Board-state golden: seed the board deterministically, dump it, and assert
+// the no-pre-existing-match invariant (zero horizontal/vertical 3-in-a-rows).
+// The dump is locked as a snapshot so the seeded board state can't drift.
+#import "modules/std.sx";
+#import "board.sx";
+t :: #import "test.sx";
+
+SEED :: 1337;
+
+// Count every horizontal or vertical window of three consecutive same-type
+// gems. A correctly initialized board has zero. This walks the finished board
+// independently of the placement logic, so it's a real check, not a tautology.
+count_three_runs :: (b: *Board) -> s32 {
+    runs : s32 = 0;
+    for 0..BOARD_ROWS: (row) {
+        for 0..(BOARD_COLS - 2): (col) {
+            g := b.at(col, row);
+            if g == b.at(col + 1, row) and g == b.at(col + 2, row) { runs += 1; }
+        }
+    }
+    for 0..(BOARD_ROWS - 2): (row) {
+        for 0..BOARD_COLS: (col) {
+            g := b.at(col, row);
+            if g == b.at(col, row + 1) and g == b.at(col, row + 2) { runs += 1; }
+        }
+    }
+    runs
+}
+
+main :: () -> s32 {
+    board : Board = ---;
+    board.init(SEED);
+
+    out(board_dump(@board));
+
+    t.expect(count_three_runs(@board) == 0, "seeded board has no 3-in-a-row runs");
+    print("ok: board_init no-match invariant holds\n");
+    return 0;
+}

tests/expected/board_init.exit

@@ -0,0 +1 @@
+0

tests/expected/board_init.stdout

@@ -0,0 +1,9 @@
+RRPPOGRG
+PGPOPRRO
+YYBBYRYB
+GBYYRGGP
+OGBRRORY
+BYRRPRBG
+YOYYROBB
+OROBPPRB
+ok: board_init no-match invariant holds