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.c

@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#ifdef __ANDROID__
+#include <android/asset_manager.h>
+
+// Caller-installed AAssetManager pointer. Chess's android_main extracts
+// it from `app->activity->assetManager` (via sx-side platform module's
+// `g_android_asset_manager` global) and feeds it here once at startup.
+// Until the setter has been called, Android falls through to fopen —
+// gives a predictable "file not found" rather than a NULL-deref.
+static AAssetManager* g_aam = NULL;
+
+void sx_android_set_asset_manager(void* m) {
+    g_aam = (AAssetManager*)m;
+}
+#endif
+
+unsigned char* read_file_bytes(const char* path, int* out_size) {
+#ifdef __ANDROID__
+    if (g_aam != NULL) {
+        // AAssetManager paths are relative to the APK's `assets/`
+        // directory. Strip a leading "assets/" so callers can use the
+        // same paths across iOS/macOS/Android (those platforms read
+        // assets via `assets/...` rooted in the bundle or CWD).
+        const char* lookup = path;
+        if (strncmp(path, "assets/", 7) == 0) {
+            lookup = path + 7;
+        }
+        AAsset* a = AAssetManager_open(g_aam, lookup, AASSET_MODE_BUFFER);
+        if (a != NULL) {
+            off_t n = AAsset_getLength(a);
+            *out_size = (int)n;
+            unsigned char* buf = (unsigned char*)malloc((size_t)n);
+            if (buf != NULL) {
+                memcpy(buf, AAsset_getBuffer(a), (size_t)n);
+            }
+            AAsset_close(a);
+            return buf;
+        }
+        // Falls through to fopen — useful when assets land in the data
+        // dir via extraction or app updates.
+    }
+#endif
+    FILE* f = fopen(path, "rb");
+    if (!f) return 0;
+    fseek(f, 0, SEEK_END);
+    *out_size = (int)ftell(f);
+    fseek(f, 0, SEEK_SET);
+    unsigned char* buf = (unsigned char*)malloc(*out_size);
+    fread(buf, 1, *out_size, f);
+    fclose(f);
+    return buf;
+}

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