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/1304-ffi-objc-block-multi-arg.sx

@@ -1,6 +1,6 @@
 // `xx closure : Block` for an arbitrary closure signature.
 //
-// The stdlib (modules/std/objc_block.sx) declares hand-rolled
+// The stdlib (modules/ffi/objc_block.sx) declares hand-rolled
 // `Into(Block) for Closure() -> void` and `Closure(bool) -> void`
 // impls — the two most common Apple block shapes. Other signatures
 // need a per-shape `__block_invoke_<sig>` trampoline + `Into(Block)`
@@ -10,10 +10,10 @@
 // This test exercises the user-declared variant: signature
 // `Closure(s32, *void) -> void` (a two-arg block — not in stdlib).
 // If the impl is missing, the compiler emits a focused diagnostic
-// pointing at modules/std/objc_block.sx as the template.
+// pointing at modules/ffi/objc_block.sx as the template.
 
 #import "modules/std.sx";
-#import "modules/std/objc_block.sx";
+#import "modules/ffi/objc_block.sx";
 
 // Trampoline matching `void (^)(int, void*)` — the C ABI Apple's
 // runtime calls. Forwards through to the sx closure with the