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/1607-platform-uikit-app.sx

@@ -12,13 +12,17 @@
 // drives the iOS run loop.
 
 #import "modules/std.sx";
-#import "modules/std/uikit.sx";
+#import "modules/ffi/objc.sx";
+
+#framework "UIKit";
+
+UIApplicationMain :: (argc: s32, argv: *void, principal_class: *NSString, delegate_class: *NSString) -> s32 #foreign;
 
 // IMP for application:didFinishLaunchingWithOptions:
 //   Obj-C: -(BOOL)application:(UIApplication *)app didFinishLaunchingWithOptions:(NSDictionary *)opts
 //   Type encoding: "c@:@@"  -- BOOL (signed char), self, _cmd, id, id
 did_finish_launching :: (self: *void, _cmd: *void, app: *void, opts: *void) -> u8 callconv(.c) {
-    NSLog(ns_string("[sx] application:didFinishLaunchingWithOptions: called\n".ptr));
+    NSLog(xx "[sx] application:didFinishLaunchingWithOptions: called\n");
     return 1; // YES
 }
 
@@ -37,5 +41,5 @@ main :: () -> s32 {
     objc_registerClassPair(SxAppDelegate);
 
     // Hand off to the iOS run loop. Never returns under normal operation.
-    return UIApplicationMain(0, xx 0, xx 0, ns_string("SxAppDelegate".ptr));
+    return UIApplicationMain(0, xx 0, xx 0, xx "SxAppDelegate");
 }