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_payment_admission.rs

@@ -0,0 +1,44 @@
+//! Outermost cross-rail admission, before wallet or payment journals.
+use anyhow::Result;
+use sha2::{Digest, Sha256};
+use std::{fs, path::Path};
+pub(crate) struct Guard {
+    _file: fs::File,
+}
+pub(crate) async fn lock(data: &Path, buyer: &str, seller: &str, content: &str) -> Result<Guard> {
+    let root = data.join("content-payment-admission");
+    let key = hex::encode(Sha256::digest(serde_json::to_vec(&(
+        buyer, seller, content,
+    ))?));
+    tokio::task::spawn_blocking(move || {
+        use std::os::{
+            fd::AsRawFd,
+            unix::fs::{OpenOptionsExt, PermissionsExt},
+        };
+        fs::create_dir_all(&root)?;
+        anyhow::ensure!(
+            fs::symlink_metadata(&root)?.is_dir(),
+            "Invalid payment admission directory"
+        );
+        fs::set_permissions(&root, fs::Permissions::from_mode(0o700))?;
+        let file = fs::OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .mode(0o600)
+            .custom_flags(libc::O_NOFOLLOW | libc::O_NONBLOCK)
+            .open(root.join(key))?;
+        anyhow::ensure!(file.metadata()?.is_file(), "Invalid payment admission lock");
+        loop {
+            if unsafe { libc::flock(file.as_raw_fd(), libc::LOCK_EX) } == 0 {
+                break;
+            }
+            let error = std::io::Error::last_os_error();
+            if error.kind() != std::io::ErrorKind::Interrupted {
+                return Err(error.into());
+            }
+        }
+        Ok(Guard { _file: file })
+    })
+    .await?
+}