refactor(x-media): drop the per-site fns the Site defaults already cover

twitter, bsky, misskey and bilibili each carried enabled() -> true,
media_headers(url) -> None and is_retryable(err) -> the trait's own default,
with no caller outside their tests (the adapters never override those
methods, so the default was already the production policy). The tests that
only restated the default are gone; the two that pin site-specific classes
(bsky's MediaPrep, bilibili's risk-control codes) now ask the Site impl, and
site/mod.rs keeps one assertion of the shared retry policy. The live
verification notes (no Referer needed for hdslb/twimg) survive as comments.

crates/x-media/src/site/bsky/interface.rs

@@ -30,10 +30,6 @@ pub static PATTERN: LazyLock<Regex> = LazyLock::new(|| {
     Regex::new(r"^(?:https?://)?bsky\.app/profile/([\w.\-:]+)/post/([\w.\-~]+)").unwrap()
 });
 
-pub fn enabled() -> bool {
-    true
-}
-
 pub async fn fetch_from_url(url: &str) -> Result<Fetched, FetchError> {
     let caps = PATTERN.captures(url).ok_or(FetchError::NotFound)?;
     let handle = caps
@@ -112,17 +108,6 @@ pub fn cache_key(url: &str) -> Option<String> {
         .map(|caps| format!("bsky:{}/{}", &caps[1], &caps[2]))
 }
 
-/// Bluesky's fetch-retry policy: transient classes only. Not-found, blocked
-/// and parse failures are permanent.
-pub fn is_retryable(err: &FetchError) -> bool {
-    matches!(err, FetchError::Http(_) | FetchError::Transient(_))
-}
-
-/// bsky media (cdn.bsky.app) needs no extra headers.
-pub fn media_headers(_url: &str) -> Option<Vec<(&'static str, String)>> {
-    None
-}
-
 /// Segments fetched (and written) at once while remuxing an HLS video. Small
 /// on purpose: a segment can be up to 20 MiB and the whole playlist is capped
 /// at 256 MiB, so this is also what bounds the remux's peak memory.
@@ -499,10 +484,11 @@ mod tests {
     /// ([`fetch_hls`]). The classes below are the ones still retried there.
     #[test]
     fn media_prep_failure_is_not_retried() {
-        assert!(!is_retryable(&FetchError::MediaPrep(
+        use crate::site::Site as _;
+        assert!(!BskySite.is_retryable(&FetchError::MediaPrep(
             "bsky video remux failed: segment 400: 503".into()
         )));
-        assert!(is_retryable(&FetchError::Transient("429".into())));
+        assert!(BskySite.is_retryable(&FetchError::Transient("429".into())));
     }
 
     #[test]

crates/x-media/src/site/bsky/mod.rs

@@ -1,6 +1,4 @@
 mod interface;
 mod model;
 
-pub use interface::{
-    BskySite, PATTERN, Post, cache_key, enabled, fetch_from_url, is_retryable, media_headers,
-};
+pub use interface::{BskySite, PATTERN, Post, cache_key, fetch_from_url};