Add durable buyer-bound Lightning recovery and explicit native retry

Preserve original invoice preimages, private snapshots and exposure provenance; serialize rail admission and retire native-only failures before explicit replacement. Qualify 55 focused UI tests and vue-tsc. Expanded 17 engine cases and combined backend acceptance remain pending; six earlier engine cases passed in isolation. No live payment or publication.

core/archipelago/src/content_purchase_download.rs

@@ -79,7 +79,7 @@ pub(crate) async fn cache(
     Ok(owned)
 }
 
-fn verified_stream<S, E>(
+pub(crate) fn verified_stream<S, E>(
     stream: S,
     expected_hash: String,
     expected_size: u64,