fix(mux)!: make the catalog Catalog non_exhaustive

`moq_mux::catalog::hang::Catalog<E>` is published (moq-mux 0.9.8) with all-`pub`
fields and no `#[non_exhaustive]`, so adding the `json` and `binary` sections
stops any downstream `Catalog { video, audio, ext }` literal from compiling.

Its sibling `hang::catalog::Catalog` already carries the attribute for exactly
this reason. Add it here so the break happens once rather than again on the next
base section. Nothing in the workspace builds one by literal, and `Default` plus
field assignment already covers every in-tree construction.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: kixelated

rs/moq-mux/src/catalog/hang/ext.rs

@@ -94,8 +94,13 @@ impl Extra {
 /// catalog derefs to the extension so its sections are reachable directly too (`catalog.scte35`, or
 /// `catalog.ext.scte35` explicitly). A consumer reading a different extension (or none) ignores
 /// sections it doesn't know.
+///
+/// Marked `#[non_exhaustive]` so a future base section can be added without breaking callers, which
+/// is what [`hang::catalog::Catalog`] already does. Build one with
+/// [`default`](Default::default) and set the fields you need.
 #[derive(Serialize, Deserialize, Clone, Default, Debug, PartialEq)]
 #[serde(bound(serialize = "E: Serialize", deserialize = "E: DeserializeOwned"))]
+#[non_exhaustive]
 pub struct Catalog<E: CatalogExt = ()> {
 	#[serde(default)]
 	pub video: hang::catalog::Video,