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/std.sx

@@ -36,7 +36,7 @@ Source_Location :: struct {
 }
 string :: []u8 #builtin;
 
-#import "std/mem.sx";
+#import "modules/std/mem.sx";
 
 // --- Allocator protocol (impls live in std/mem.sx) ---
 
@@ -492,9 +492,9 @@ List :: struct ($T: Type) {
 }
 // --- The stdlib namespace tail: flat-importing std.sx carries these ---
 
-mem :: #import "std/mem.sx";
-xml :: #import "std/xml.sx";
-log :: #import "std/log.sx";
+mem :: #import "modules/std/mem.sx";
+xml :: #import "modules/std/xml.sx";
+log :: #import "modules/std/log.sx";
 // fs/process/socket/json/cli/hash/test join the tail once the global
 // last-wins maps are fully own-wins (their decls collide across programs
 // when pulled into every closure) — import them directly meanwhile: