ci+fixtures: UCRT64 msys2 env; fix remaining fixture paths

ci (Windows): the runner's msys2 shell starts as the base MSYS
environment (MSYSTEM=MSYS), which install-deps.sh rejects. Set the
job-level MSYSTEM=UCRT64 env and prepend /ucrt64/bin to PATH in every
Windows step, so pacman installs and the toolchain resolve against the
mingw-w64-ucrt-x86_64 packages.

fixtures: the restructured workspace moved the media fixtures into
crates/oak-app/tests; the remaining references pointed at the old
repo-root tests/ — oak-codec realmedia_tests + hwdecode, oak-node
serializer golden, oak-worker procpool integration. All point at
../oak-app/tests now (oak-app's own tests/demo.mp4 references were
already correct after the move).

crates/oak-codec/src/hwdecode.rs

@@ -229,7 +229,7 @@ mod tests {
 	#[cfg(target_os = "macos")]
 	#[test]
 	fn videotoolbox_device_opens_for_h264() {
-		let mut input = ffmpeg::format::input(&"../../tests/demo.mp4".to_string())
+		let mut input = ffmpeg::format::input(&"../oak-app/tests/demo.mp4".to_string())
 			.expect("open demo.mp4");
 		let fstream = input.stream(0).expect("stream 0");
 		let params = fstream.parameters();

crates/oak-codec/src/realmedia_tests.rs

@@ -40,7 +40,7 @@ use std::sync::Arc;
 
 /// `tests/demo.mp4` at the repository root.
 fn demo_path() -> std::path::PathBuf {
-	std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("../../tests/demo.mp4")
+	std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("../oak-app/tests/demo.mp4")
 }
 
 fn video_params(stream: CodecStream, time: Rational) -> RetrieveVideoParams {