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/math/scalar.sx

@@ -0,0 +1,37 @@
+PI :f32: 3.14159265;
+TAU :f32: 6.28318530;
+DEG2RAD :f32: 0.01745329;
+RAD2DEG :f32: 57.2957795;
+
+sqrt :: (x: $T) -> T #builtin;
+sin :: (x: $T) -> T #builtin;
+cos :: (x: $T) -> T #builtin;
+floor :: (x: $T) -> T #builtin;
+
+min :: (a: $T, b: T) -> T {
+    if a < b then a else b
+}
+
+max :: (a: $T, b: T) -> T {
+    if a > b then a else b
+}
+
+clamp :: (val: $T, lo: T, hi: T) -> T {
+    if val < lo then lo
+    else if val > hi then hi
+    else val
+}
+
+abs :: (x: $T) -> T {
+    if x < 0 then 0 - x else x
+}
+
+lerp :: (a: f32, b: f32, t: f32) -> f32 {
+    a + (b - a) * t
+}
+
+sign :: (x: $T) -> T {
+    if x > 0 then 1
+    else if x < 0 then 0 - 1
+    else 0
+}