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/0829-packs-param-impl-mixed-pack-source-ambiguous.sx

@@ -12,7 +12,7 @@
 // Fail-before (pre-E6BR-4): the wrapped/pack source fell to the no-author
 // `type_bridge.resolveTemplateSignatureType` wrapper (global last-wins, no
 // diagnostic), so the `*Box` collision registered silently. Protects the pure-pack
-// `Closure(..$args) -> $R` bridge in `library/modules/std/objc_block.sx` (0504 /
+// `Closure(..$args) -> $R` bridge in `library/modules/ffi/objc_block.sx` (0504 /
 // 1302 / 1304 stay byte-identical), whose single author keeps the prefix-free path.
 
 #import "modules/std.sx";