ci: probe OFX plugin discovery with a real fixture plugin; cd: restore Windows NSIS job

- new minimal C OFX plugin fixture (ci_test_plugin.c) compiled into a
  real .ofx.bundle by build_fixture.sh; the CI step points
  OFX_PLUGIN_PATH at it and asserts the scan_probe example discovers
  AND registers it (Linux/macOS)
- host bundle binary search now also covers the OFX-standard Win64
  platform directory
- cd.yml: restore the Windows NSIS packaging job (obsolete oakengine
  cdylib prebuild dropped) and repair the job indentation that had
  silently detached the macos/release jobs from the jobs: map;
  releases now gate on all three platforms

crates/oakplugin/src/host.rs

@@ -640,6 +640,8 @@ fn find_binary_in_bundle(bundle: &Path) -> Option<PathBuf> {
 		contents.join("MacOS"),
 		contents.join("Linux-x86-64"),
 		contents.join("Linux-aarch64"),
+		// OFX 规范的 64 位 Windows 平台目录。
+		contents.join("Win64"),
 	] {
 		if let Ok(entries) = std::fs::read_dir(&platform_dir) {
 			for e in entries.flatten() {