perf(cashu): avoid needless wallet-key decrypt and secret parse on redeem

Follow-up to the P2PK redeem support:

- Gate the redeem signing-key gathering behind an actual P2PK lock. The
  wallet P2PK key is decrypted from kind:17375 via the signer — a network
  round-trip on a NIP-46 bunker (and a possible approval prompt). The common
  case (a plain, unlocked token) needs none of it, so only fetch keys when
  `anyP2pkLocked()` is true. Applies to both the wallet ViewModel and the amy
  CLI token-redeem path.
- Fast-reject in `P2PK.parseSecret`: NUT-10 well-known secrets are JSON
  arrays, so bail before the throwing JSON parse when the string isn't one.
  Redeem parses every proof's secret once, so this drops a thrown+caught
  exception per plain proof (also benefits the nutzap redeem path).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QKeRaX749TYnJ7oR8UpqA4

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/wallet/CashuWalletViewModel.kt

@@ -33,6 +33,7 @@ import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
 import com.vitorpamplona.quartz.lightning.LnInvoiceUtil
 import com.vitorpamplona.quartz.nip60Cashu.mintApi.MeltQuoteBolt11ResponseDto
 import com.vitorpamplona.quartz.nip60Cashu.mintApi.MintHttpException
+import com.vitorpamplona.quartz.nip60Cashu.p2pk.anyP2pkLocked
 import com.vitorpamplona.quartz.nip60Cashu.token.CashuTokenB64Parser
 import com.vitorpamplona.quartz.nip87Ecash.recommendation.MintRecommendationEvent
 import com.vitorpamplona.quartz.utils.Log
@@ -909,7 +910,15 @@ class CashuWalletViewModel : ViewModel() {
                 // Keys that can unlock a P2PK-locked token: our wallet key, and
                 // — for a local nsec login only — the identity key (some senders,
                 // e.g. Bey Wallet, P2PK-lock ecash straight to the recipient npub).
-                val (walletKey, identityKey) = state.redeemSigningKeys()
+                // Only gathered when a proof is actually locked: redeemSigningKeys()
+                // decrypts the kind:17375 privkey, which is a signer round-trip on
+                // a bunker/external signer we shouldn't pay for a plain token.
+                val (walletKey, identityKey) =
+                    if (parsedTokens.any { it.proofs.anyP2pkLocked() }) {
+                        state.redeemSigningKeys()
+                    } else {
+                        null to null
+                    }
                 val total =
                     parsedTokens.sumOf {
                         ops

cli/src/main/kotlin/com/vitorpamplona/amethyst/cli/commands/cashu/CashuReceiveCommands.kt

@@ -29,6 +29,7 @@ import com.vitorpamplona.quartz.lightning.LnInvoiceUtil
 import com.vitorpamplona.quartz.nip01Core.core.toHexKey
 import com.vitorpamplona.quartz.nip01Core.signers.NostrSignerInternal
 import com.vitorpamplona.quartz.nip60Cashu.p2pk.P2PKUnredeemableException
+import com.vitorpamplona.quartz.nip60Cashu.p2pk.anyP2pkLocked
 import com.vitorpamplona.quartz.nip60Cashu.token.CashuTokenB64Parser
 
 /**
@@ -162,9 +163,12 @@ object CashuReceiveCommands {
                 // Keys that can unlock a P2PK-locked token: the wallet's kind:17375
                 // key, plus — for a local key account — the identity key (some
                 // senders, e.g. Bey Wallet, P2PK-lock ecash to the recipient npub).
-                val snap = ctx.cashuSnapshot()
-                val walletKey = snap.walletEvent?.let { runCatching { it.privkey(ctx.signer) }.getOrNull() }
-                val identityKey = (ctx.signer as? NostrSignerInternal)?.keyPair?.privKey?.toHexKey()
+                // Only decrypt the wallet key when a proof is actually locked (a
+                // signer round-trip on bunker accounts, wasted on a plain token).
+                val locked = parsed.any { it.proofs.anyP2pkLocked() }
+                val walletKey =
+                    if (locked) ctx.cashuSnapshot().walletEvent?.let { runCatching { it.privkey(ctx.signer) }.getOrNull() } else null
+                val identityKey = if (locked) (ctx.signer as? NostrSignerInternal)?.keyPair?.privKey?.toHexKey() else null
                 var total = 0L
                 var lastTokenEventId: String? = null
                 var lastHistoryEventId: String? = null

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip60Cashu/p2pk/P2PK.kt

@@ -92,8 +92,13 @@ object P2PK {
      * Parse a Cashu P2PK secret string. Returns null if the string is not a
      * NUT-11 P2PK secret.
      */
-    fun parseSecret(secret: String): ParsedP2pk? =
-        try {
+    fun parseSecret(secret: String): ParsedP2pk? {
+        // Fast reject before the (throwing) JSON parse: NUT-10 well-known
+        // secrets are a JSON array (`["P2PK", …]`), while a plain Cashu secret
+        // is opaque hex/base64. Redeeming a token calls this once per proof, so
+        // skipping the parse+exception for the common plain case matters.
+        if (!secret.looksLikeJsonArray()) return null
+        return try {
             val arr = json.parseToJsonElement(secret) as? JsonArray ?: return null
             if (arr.size < 2) return null
             val kind = (arr[0] as? JsonPrimitive)?.content
@@ -105,6 +110,16 @@ object P2PK {
         } catch (_: Exception) {
             null
         }
+    }
+
+    /** True when the first non-whitespace char is `[` — i.e. it may be a JSON array. */
+    private fun String.looksLikeJsonArray(): Boolean {
+        for (c in this) {
+            if (c.isWhitespace()) continue
+            return c == '['
+        }
+        return false
+    }
 
     /**
      * BIP-340 Schnorr signature over `sha256(secret_bytes)` used as the unlock