std: restructure step 3 — ffi/ moves, build.sx, math dir spelling, fixtures

- objc.sx, objc_block.sx (from std/) + sdl3/opengl/raylib/stb/stb_truetype/
  wasm vendor bindings (from modules/ root) -> modules/ffi/
- std/uikit.sx deleted: platform/uikit.sx already declares UIApplicationMain
  and imports objc; '#framework "UIKit"' cannot live in a file imported on
  macOS targets (unconditional link directive, UIKit is iOS-only), so the
  three iOS-only examples carry the 3-line glue inline. 1607/1608/1616 also
  un-rotted (dead ns_string -> 'xx "..."' Into conversions, callconv(.c)
  msgSend fn-ptrs) — all three build for ios-sim/ios again.
- math/math.sx -> math/scalar.sx; one spelling '#import "modules/math"'
  everywhere (4 pinned IR snapshots regenerated: dir import adds Vec2/Mat4
  to the type tables).
- compiler.sx -> build.sx (imports, CLAUDE.md bundling table, specs.md).
- testpkg/ + test_c.sx -> tests/fixtures/ (resolve CWD-relative from repo
  root, same as vendors/).
- library-internal imports use full modules/... paths (std.sx tail,
  platform/bundle.sx, fixtures).

library/modules/ffi/raylib.sx

@@ -0,0 +1,17 @@
+raylib :: #library "raylib";
+
+Color :: struct {
+    r, g, b, a: u8;
+}
+
+Vector2 :: struct {
+    x, y: f32;
+}
+
+InitWindow :: (width: s32, height: s32, title: [:0]u8) -> void #foreign raylib;
+CloseWindow :: () -> void #foreign raylib;
+WindowShouldClose :: () -> bool #foreign raylib;
+BeginDrawing :: () -> void #foreign raylib;
+EndDrawing :: () -> void #foreign raylib;
+ClearBackground :: (color: Color) -> void #foreign raylib;
+DrawTriangle :: (v1: Vector2, v2: Vector2, v3: Vector2, color: Color) -> void #foreign raylib;