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).

examples/expected/0201-generics-generic-struct.ir

@@ -2252,9 +2252,6 @@ declare i64 @write(i32, ptr, i64)
 ; Function Attrs: nounwind
 declare void @log_emit(ptr, ptr, ptr) #0
 
-; Function Attrs: nounwind
-declare float @lerp(ptr, float, float, float) #0
-
 ; Function Attrs: nounwind
 declare void @Mat4.identity(ptr sret({ [16 x float] }), ptr) #0
 
@@ -2285,6 +2282,9 @@ declare void @Mat4.ortho(ptr sret({ [16 x float] }), ptr, float, float, float, f
 ; Function Attrs: nounwind
 declare void @Mat4.perspective(ptr sret({ [16 x float] }), ptr, float, float, float, float) #0
 
+; Function Attrs: nounwind
+declare float @lerp(ptr, float, float, float) #0
+
 ; Function Attrs: nounwind
 declare { float, float } @Vec2.zero(ptr) #0