fix(ecash): give every node a backup phrase, and prove restore works

Running the route suite on this box surfaced that the backup was
unreachable here: `identity/master_seed.enc` is written during
onboarding, and any node onboarded before that step existed simply does
not have one. Reveal bailed with "this node has no encrypted seed
backup", and restore followed it down.

But the choice on such a node was never "derived phrase or independent
phrase" — it was "independent phrase or no backup at all", and a wallet
whose coins can be restored from words the operator holds beats one
whose coins die with a single file. So it now generates one, recorded as
`independent`, and every surface that shows it says plainly that
restoring the node will not bring the ecash back — only these words
will. `derivable_from_node_seed` lets the card say which kind you are
about to get *before* you write anything down.

Also: a mint that never implemented NUT-09 answered restore with a bare
404, which surfaced as "mint returned 404 with no further detail" —
true, and useless to someone trying to get their coins back. It now
names the limitation.

The route suite was reading `result.amount_sats` from mint-claim, which
answers with `minted_sats`. A working claim had been reporting as a
failure; that was one of the two reds carried over from yesterday.

The real gap, though, was that "recovered 0 sats" passes on a wallet
with nothing to find — exactly the shape of a backup that looks fine
until the day you need it. test-ecash-restore.sh does the test that
settles it: mint, **delete the wallet file**, restore, check the coins
came back. On this box: 87 sats before the wipe, 0 after, 61 recovered
from the phrase alone — every coin minted since the phrase existed, and
none of the 26 sats minted before it, which used random secrets and
never could come back. Testnet only, and it refuses to run otherwise.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/archipelago/src/wallet/mint_client.rs

@@ -648,6 +648,18 @@ impl MintClient {
 
         if !res.status().is_success() {
             let status = res.status();
+            // NUT-09 is optional. A mint that never implemented it answers 404
+            // or 405, which `mint_error` would render as "mint returned 404
+            // with no further detail" — true, and useless to someone trying to
+            // get their coins back. Name the actual limitation instead.
+            if matches!(status.as_u16(), 404 | 405 | 501) {
+                anyhow::bail!(
+                    "This mint does not support restoring from a backup phrase (NUT-09). \
+                     Your coins are safe, but they can only be recovered from a wallet \
+                     file backup while they stay at {}",
+                    self.url
+                );
+            }
             let body = res.text().await.unwrap_or_default();
             return Err(mint_error("Restore", status, &body));
         }

core/archipelago/src/wallet/nut13.rs

@@ -215,6 +215,38 @@ pub async fn establish_from_master(
     Ok(EcashSeed::from_mnemonic(derived, SeedSource::NodeSeed))
 }
 
+/// Establish a wallet seed that is **not** derived from the node's master
+/// seed, for a node that has no encrypted master seed to derive from.
+///
+/// Plenty of nodes are in that position: `identity/master_seed.enc` is written
+/// during onboarding, and any node onboarded before that step existed simply
+/// does not have one. The choice there is not "derived phrase or independent
+/// phrase" — it is "independent phrase or **no backup at all**", and a wallet
+/// whose coins can be restored from words the operator holds is strictly
+/// better than one whose coins die with a single file.
+///
+/// The cost is stated plainly rather than hidden: the phrase is recorded as
+/// [`SeedSource::Independent`], and every surface that shows it says that
+/// restoring the node will *not* bring this wallet back — only these words
+/// will. That is a real obligation on the operator, so it must never be the
+/// silent default when derivation was possible; [`establish_from_master`] is
+/// what a node with a master seed gets.
+pub async fn establish_independent(data_dir: &Path) -> Result<EcashSeed> {
+    if let Some(existing) = load_seed(data_dir).await? {
+        return Ok(existing);
+    }
+    // Same guarded generation path as the node's own seed: a named CSPRNG and
+    // the degenerate-entropy check, not a dependency's default (KEY-05).
+    let (mnemonic, _seed) = crate::seed::MasterSeed::generate()?;
+    write_seed(data_dir, &mnemonic, SeedSource::Independent).await?;
+    warn!(
+        "Established an INDEPENDENT ecash backup phrase: this node has no encrypted \
+         master seed to derive one from, so restoring the node will not restore this \
+         ecash wallet — only the phrase itself will."
+    );
+    Ok(EcashSeed::from_mnemonic(mnemonic, SeedSource::Independent))
+}
+
 /// Write the seed file at 0600, creating the wallet directory if needed.
 async fn write_seed(
     data_dir: &Path,