refactor(site): drop site_id_from_key, split the key at its caller

site_id_from_key split the prefix out and then walked the registry to confirm the prefix was a registered site id — a round trip over a value cache_key itself produced from that registry: the unknown and no-colon branches were unreachable for any key the bot makes. The single caller (the link-cache hit path) splits the prefix directly; the registry-echo test and the misskey assertion of it go with it.

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

@@ -209,7 +209,6 @@ mod tests {
             cache_key("https://misskey.io/notes/aotihl10lqrs015s"),
             Some("misskey:aotihl10lqrs015s".to_string())
         );
-        assert_eq!(crate::site::site_id_from_key("misskey:abc"), "misskey");
     }
 
     #[test]

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

@@ -231,19 +231,6 @@ pub fn cache_key(url: &str) -> Option<String> {
     SITES.iter().find_map(|site| site.cache_key(url))
 }
 
-/// The site id carried by a cache key (`"twitter:123"` → `"twitter"`).
-/// Unknown prefixes fall back to `"unknown"`. The bot uses this on the
-/// link-cache hit path, where no [`Fetched`] is available — the same value
-/// a fresh fetch would read from [`Fetched::site_id`].
-pub fn site_id_from_key(key: &str) -> &'static str {
-    let prefix = key.split(':').next().unwrap_or("");
-    SITES
-        .iter()
-        .map(|site| site.id())
-        .find(|id| *id == prefix)
-        .unwrap_or("unknown")
-}
-
 #[derive(Debug, Error)]
 pub enum FetchError {
     #[error("http error: {0}")]
@@ -591,16 +578,6 @@ mod tests {
         assert_eq!(cache_key("https://example.com/not-a-post"), None);
     }
 
-    #[test]
-    fn site_id_from_key_parses_prefix() {
-        assert_eq!(site_id_from_key("twitter:123"), "twitter");
-        assert_eq!(site_id_from_key("pixiv:123"), "pixiv");
-        assert_eq!(site_id_from_key("bsky:handle.example/3lorem"), "bsky");
-        assert_eq!(site_id_from_key("bilibili:123"), "bilibili");
-        assert_eq!(site_id_from_key("unknown:1"), "unknown");
-        assert_eq!(site_id_from_key("no-colon"), "unknown");
-    }
-
     #[test]
     fn registry_lists_all_sites_in_dispatch_order() {
         assert_eq!(

crates/xmedia-bot/src/handlers/urls.rs

@@ -554,8 +554,10 @@ async fn url_media_inner(
         log::debug!("link cache hit for {key}");
         let chat_data = ctx.chat_store.get(chat_id).await;
         // Cache keys are prefixed with the site id ("twitter:…"), matching
-        // the value a fresh fetch would read from Fetched::site_id.
-        let site = x_media::site::site_id_from_key(&key);
+        // the value a fresh fetch would read from Fetched::site_id. The key
+        // came out of cache_key, so its prefix is a registered id by
+        // construction — splitting it off is the whole lookup.
+        let site = key.split(':').next().unwrap_or("");
         let format = chat_data.format_for(site);
         // One call for both: `caption_from_fields` returns the truncated
         // built-in caption itself when the chat has no format for this site.