fix: CI fallout from the i18n refactor + Windows path assertions

examples/screenshot.rs: migrate to language_code()/set_language_code
(the Language enum is gone); only CI's example build caught it — local
--lib runs never compile examples.

oakcodec tests: build path expectations with Path::join instead of
'/'-joined literals — production uses platform-native separators, so
the derivation assertions failed on Windows ("dir\img007.jpg" vs
"dir/img007.jpg"). Behaviour unchanged; the tests were never reached
on Windows before (earlier failures aborted the run first).

oakui::ofx interact test: poll up to 5s for the destroy marker record
before asserting. The active-interact slot is process-global; a
concurrent viewer frame sync from another test's real engine can take
the interact out of the slot and be preempted between take and
destroy, so the plugin's destroy record occasionally lands a few
milliseconds after this test removed the marker env var (Linux CI:
"lifecycle actions missing: [...]" with everything but destroy
present).

crates/oakcodec/src/decoder.rs

@@ -629,7 +629,10 @@ mod tests {
 		);
 		assert_eq!(
 			transform_image_sequence_file_name("dir/img012.jpg", 7),
-			"dir/img007.jpg"
+			// Path::join separators are platform-native (\ on Windows).
+			std::path::Path::new("dir")
+				.join("img007.jpg")
+				.to_string_lossy()
 		);
 		// No digit run: number appended with no padding (C++ behavior).
 		assert_eq!(