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/kb_text_shape/kb/LICENSE

@@ -0,0 +1,19 @@
+zlib License
+
+(C) Copyright 2024-2025 Jimmy Lefevre
+
+This software is provided 'as-is', without any express or implied
+warranty.  In no event will the authors be held liable for any damages
+arising from the use of this software.
+
+Permission is granted to anyone to use this software for any purpose,
+including commercial applications, and to alter it and redistribute it
+freely, subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not
+  claim that you wrote the original software. If you use this software
+  in a product, an acknowledgment in the product documentation would be
+  appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be
+  misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.

vendors/kb_text_shape/kb_text_shape_impl.c

@@ -0,0 +1,2 @@
+#define KB_TEXT_SHAPE_IMPLEMENTATION
+#include "kb/kb_text_shape.h"

vendors/kb_text_shape/kbts_api.h

@@ -0,0 +1,15 @@
+// Minimal API declarations for SX import.
+// Only the functions/types we actually use — avoids parsing the full 30k-line header.
+
+typedef struct kbts_shape_context kbts_shape_context;
+typedef struct kbts_font kbts_font;
+
+kbts_shape_context *kbts_CreateShapeContext(void *Allocator, void *AllocatorData);
+void kbts_DestroyShapeContext(kbts_shape_context *Context);
+kbts_font *kbts_ShapePushFontFromMemory(kbts_shape_context *Context, void *Memory, int Size, int FontIndex);
+void kbts_GetFontInfo2(kbts_font *Font, void *Info);
+void kbts_ShapeBegin(kbts_shape_context *Context, unsigned int ParagraphDirection, unsigned int Language);
+void kbts_ShapeUtf8(kbts_shape_context *Context, const char *Utf8, int Length, unsigned int UserIdGenerationMode);
+void kbts_ShapeEnd(kbts_shape_context *Context);
+int kbts_ShapeRun(kbts_shape_context *Context, void *Run);
+int kbts_GlyphIteratorNext(void *It, void **Glyph);