stdlib: relocate modules under library/

- examples/modules/ -> library/modules/ (top-level, no more
  symlink hacks in consumer projects)
- compiler discovers stdlib via _NSGetExecutablePath / readlink
  /proc/self/exe; searches dev layout (../../library), install
  layout (../library), and alongside-binary fallback
- SX_STDLIB_PATH env var overrides for tests / dev convenience
- SX_DEBUG_STDLIB env var dumps the discovery results
- build.zig installs library/ alongside the binary
- Compilation gains stdlib_paths field threaded through resolveImports
- 50 tests pass; consumer projects can now build from any cwd

library/modules/std/uikit.sx

@@ -0,0 +1,17 @@
+// UIKit framework bindings — iOS only.
+//
+// Consumers `#import "modules/std/uikit.sx";` and inherit the
+// `#framework "UIKit"` link directive plus any C-API declarations exposed
+// here. Obj-C class/method dispatch goes through `modules/std/objc.sx`
+// which we re-import so users only need this one file.
+
+#import "objc.sx";
+
+#framework "UIKit";
+
+// int UIApplicationMain(int argc, char *_Nullable argv[_Nonnull],
+//                       NSString *_Nullable principalClassName,
+//                       NSString *_Nullable delegateClassName);
+//
+// Blocks. Drives the iOS run loop. Normally never returns.
+UIApplicationMain :: (argc: s32, argv: *void, principal_class: *void, delegate_class: *void) -> s32 #foreign;