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/raylib.sx

@@ -0,0 +1,17 @@
+raylib :: #library "raylib";
+
+Color :: struct {
+    r, g, b, a: u8;
+}
+
+Vector2 :: struct {
+    x, y: f32;
+}
+
+InitWindow :: (width: s32, height: s32, title: [:0]u8) -> void #foreign raylib;
+CloseWindow :: () -> void #foreign raylib;
+WindowShouldClose :: () -> bool #foreign raylib;
+BeginDrawing :: () -> void #foreign raylib;
+EndDrawing :: () -> void #foreign raylib;
+ClearBackground :: (color: Color) -> void #foreign raylib;
+DrawTriangle :: (v1: Vector2, v2: Vector2, v3: Vector2, color: Color) -> void #foreign raylib;