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).

src/ir/lower/coerce.zig

@@ -117,7 +117,7 @@ pub fn lowerXX(self: *Lowering, operand: Ref, operand_node: *const Node) Ref {
 /// can emit a focused diagnostic when no `Into(Block) for Closure(...)`
 /// impl is reachable. Replaces what was briefly a compiler-synthesised
 /// trampoline path with a "declare an impl" requirement — the stdlib
-/// covers common signatures (see modules/std/objc_block.sx), users
+/// covers common signatures (see modules/ffi/objc_block.sx), users
 /// add their own for unusual ones.
 pub fn isClosureToBlockCast(self: *Lowering, src_ty: TypeId, dst_ty: TypeId) bool {
     if (src_ty.isBuiltin()) return false;
@@ -251,7 +251,7 @@ pub fn tryUserConversion(self: *Lowering, operand: Ref, operand_node: *const Nod
                 const saved = diags.current_source_file;
                 diags.current_source_file = operand_node.source_file orelse self.current_source_file;
                 defer diags.current_source_file = saved;
-                diags.addFmt(.err, operand_node.span, "no `Into(Block) for {s}` impl — add a per-signature `__block_invoke_<sig>` trampoline + Into impl alongside the existing ones in modules/std/objc_block.sx, or declare it in your own code", .{self.mangleTypeName(src_ty)});
+                diags.addFmt(.err, operand_node.span, "no `Into(Block) for {s}` impl — add a per-signature `__block_invoke_<sig>` trampoline + Into impl alongside the existing ones in modules/ffi/objc_block.sx, or declare it in your own code", .{self.mangleTypeName(src_ty)});
             }
             return operand;
         }