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/0206-generics-generic-into-block.sx

@@ -6,7 +6,7 @@
 //
 // This test exercises a closure shape (`Closure(s64, s64) -> void`)
 // that has NO hand-rolled `Into(Block)` impl in
-// `library/modules/std/objc_block.sx`. Before step 5.2 lands,
+// `library/modules/ffi/objc_block.sx`. Before step 5.2 lands,
 // `xx cl : Block` errors out with the "no Into(Block) for
 // cl_s64_s64__void" focused diagnostic. After the generic impl
 // lands, the same call resolves through the pack-shaped impl and
@@ -18,7 +18,7 @@
 // to its registered invoke.
 
 #import "modules/std.sx";
-#import "modules/std/objc_block.sx";
+#import "modules/ffi/objc_block.sx";
 
 g_a: s64 = 0;
 g_b: s64 = 0;