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/wasm.sx

@@ -0,0 +1,4 @@
+libc :: #library "c";
+
+emscripten_set_main_loop  :: (func: *void, fps: s32, sim_infinite: s32) #foreign libc;
+emscripten_run_script_int :: (script: [:0]u8) -> s32 #foreign libc;