youtube: fall back to VISIONOS primary when ANDROID is bot-walled

YouTube's 2026 poToken/BotGuard enforcement now rejects the pot-less ANDROID
client outright (playabilityStatus "sign in to confirm you're not a bot"), which
aborted the whole extraction since ANDROID was the required primary.

stream_info_with() now cascades ANDROID -> VISIONOS for the primary player
response: when ANDROID is unusable (fetch error, failed playability, or decoy),
retry with VISIONOS (client 101) as the primary. VISIONOS is pot-free,
visitorData-based, and needs no JS player -- the current survivor (yt-dlp
default; NewPipeExtractor #1508). Only if VISIONOS also fails do we surface the
ORIGINAL ANDROID error (keeps the real bot-wall reason visible).

The fallback drops the (poisoned) visitorData cache first so VISIONOS mints a
fresh one (the ingredient that passes attestation). On the VISIONOS-primary
path the primary source carries no poToken (null) and uses the VISIONOS cpn, so
no ANDROID pot/cpn is appended to a visionOS URL (would 403); the redundant
add-on VISIONOS fetch is skipped. Output shape is unchanged (still the muxed
itag-18 the app plays) -- no app or delivery-labeling change. 5 new unit tests.

examples/live_extract.rs

@@ -0,0 +1,50 @@
+// Throwaway live diagnostic (playback-outage triage). Not shipped.
+use std::sync::Arc;
+use strawcore_core::downloader::ReqwestDownloader;
+use strawcore_core::youtube::stream_extractor::{stream_info_with, ExtractOptions};
+use strawcore_core::NewPipe;
+
+fn client_of(url: &str) -> String {
+    url.split('&')
+        .find_map(|kv| kv.strip_prefix("c="))
+        .unwrap_or("?")
+        .to_string()
+}
+
+fn report(label: &str, vid: &str, visionos: bool) {
+    let opts = ExtractOptions {
+        fetch_visionos_client: visionos,
+        ..Default::default()
+    };
+    match stream_info_with(vid, opts) {
+        Ok(i) => {
+            let a = i.audio_streams.first().map(|s| client_of(&s.url));
+            let v = i
+                .video_streams
+                .first()
+                .or_else(|| i.video_only_streams.first())
+                .map(|s| client_of(&s.url));
+            println!(
+                "[{label}] OK  audio={} video={} video_only={} dash={} hls={} | first audio c={:?} first video c={:?}",
+                i.audio_streams.len(),
+                i.video_streams.len(),
+                i.video_only_streams.len(),
+                i.dash_manifest_url.is_some(),
+                i.hls_manifest_url.is_some(),
+                a,
+                v,
+            );
+        }
+        Err(e) => println!("[{label}] ERR {e:?}"),
+    }
+}
+
+fn main() {
+    let dl = Arc::new(ReqwestDownloader::new().expect("downloader"));
+    NewPipe::init(dl);
+    let vid = std::env::args()
+        .nth(1)
+        .unwrap_or_else(|| "dQw4w9WgXcQ".into());
+    report("visionOS ON  (shipped)", &vid, true);
+    report("visionOS OFF (like 91) ", &vid, false);
+}