P4.3: render seeded board with real gem sprites (sx, iOS sim)

Adopt the modules/ui UIPipeline framework (as the chess reference app does)
and replace the P0 placeholder quad with a BoardView (View protocol, modeled
on chess/board_view.sx):
- background.png fills the screen; an 8x8 cell.png grid is centered in the
  safe area; each cell's gem is sampled from gems.png by UV column = gem index
  (0=red .. 5=purple).
- Drive it from board.sx seeded with 1337 (the board_init golden's seed), so
  the on-screen layout matches that snapshot gem-for-gem.

main.sx now hosts the view via UIPipeline (Metal on iOS, GL on desktop) and
heap-allocates the board/asset state behind pointers (UFCS method calls on a
value-typed global mutate a copy, so mutable state must live behind a pointer
as the reference app does).

Vendor the C deps the UI module's image/font path needs (stb_image,
stb_truetype, kb_text_shape, file_utils); their #include "vendors/..." paths
resolve relative to the project root.

Evidence: ios-sim build links clean; tools/run_tests.sh 11/11 pass; running
app captured at goldens/p4_board.png.

vendors/file_utils/file_utils.h

@@ -0,0 +1,13 @@
+#ifndef FILE_UTILS_H
+#define FILE_UTILS_H
+
+unsigned char* read_file_bytes(const char* path, int* out_size);
+
+#ifdef __ANDROID__
+// Install the AAssetManager that `read_file_bytes` consults for paths
+// rooted inside the APK. Caller is responsible for passing the manager
+// from `ANativeActivity->assetManager` before any read_file_bytes call.
+void sx_android_set_asset_manager(void* m);
+#endif
+
+#endif