perf: retry the bsky HLS request, not the whole fetch

A failed bsky video remux was reported as `FetchError::Transient`, so the
fetch loop retried the *whole* adapter — master playlist, variant playlist
and up to 500 segments again (256 MiB of cap each time), for a failure that
happened near the end of the work the retry was about to redo. The message
had to stay honest (returning `Ok` with no media reads as "this post has no
media"), so it needed a class of its own.

`FetchError::MediaPrep` is that class: post fetched, media could not be
prepared locally, not retryable (the per-site `is_retryable` whitelist
excludes it by construction), with its own user-facing text — the generic
"failed to fetch" would have hidden that the download or encode was what
broke. The retry itself is not lost, it moved: `fetch_hls` retries the
request that actually failed, once, for the classes a retry can change
(transport, 429/5xx).

Checked the sibling sites before widening the change: pixiv already degrades
to no media when the ugoira encode fails (`Ok(None)`), and its frame-zip
download failure is the last step so a re-fetch re-does only that; twitter's
auth leg replays two cheap metadata GETs and a GraphQL 5xx *is* worth
retrying. Neither needed the new class.

Verified with a throwaway proxy harness: a 503 answered twice-in-a-row path
costs 2 requests and succeeds, a 404 costs exactly 1 and fails (no pointless
retry). `site::bsky::interface::tests::media_prep_failure_is_not_retried`
pins the classification.

`cargo fmt --check`, `cargo clippy --workspace --all-targets --locked -- -D
warnings` and `cargo test --workspace --locked` clean.

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

@@ -260,6 +260,13 @@ pub enum FetchError {
     /// A download exceeded the caller's size cap (see [`download_media_limited`]).
     #[error("media too large")]
     TooLarge,
+    /// The post was fetched, but its media could not be prepared locally — a
+    /// download or encode step that runs *after* the site's own response
+    /// (bsky's HLS remux, say). Deliberately not retryable: the retry would
+    /// replay the whole fetch, redoing the download work that just failed
+    /// instead of the request that failed.
+    #[error("media could not be prepared: {0}")]
+    MediaPrep(String),
     /// A transient server-side failure (429 / 5xx); [`fetch`] retries these.
     #[error("transient: {0}")]
     Transient(String),