fix(wallet/pq): satisfy --strict exhaustive-switch narrowing

The switch in walletSchemeName covers every WalletSchemeID enum
value, so TypeScript --strict narrows the default arm's `s` to
`never` and rejects `s.toString(16)`. Cast through `number` for
the forward-compat fallback path.

No behaviour change; KAT vector for AccountID derivation
(chainID=9000, scheme=0x42) still matches the Go side at
76a03630148103ec558cf4d8f7e8a2d8766ea205d96a4529249c3aaf9c5d078cc9f9fdb8f21a5e7ef6bb4c4ea29c9654.

pkgs/wallet/src/features/wallet/pq/pqAccount.ts

@@ -56,8 +56,14 @@ export function walletSchemeName(s: WalletSchemeID): string {
       return 'ml-dsa-65'
     case WalletSchemeID.MLDSA87:
       return 'ml-dsa-87'
-    default:
-      return `wallet-scheme(0x${s.toString(16).padStart(2, '0')})`
+    default: {
+      // Forward-compat: a numeric scheme byte that does not match any
+      // known enum case still gets a readable name. The cast to number
+      // is required because the switch above is exhaustive over the
+      // enum and TypeScript narrows `s` to `never` here.
+      const raw = s as number
+      return `wallet-scheme(0x${raw.toString(16).padStart(2, '0')})`
+    }
   }
 }