feat(wallet): enforce confirmation thresholds before spending UTXOs

Coinbase outputs require COINBASE_MATURITY + 1 = 121 confirmations before
they can be spent. The consensus rule (palladiumcore tx_verify.cpp) is
nSpendHeight - nHeight >= 120; the +1 adds one block of safety margin,
matching the Qt wallet (wallet.cpp: COINBASE_MATURITY+1). Regular UTXOs
require MinConfirmations (6 on mainnet, 1 on testnet/regtest — wallet
policy, no consensus rule).

Changes:
- ChainProfile: add CoinbaseMaturity and MinConfirmations fields
- ChainProfiles: mainnet CoinbaseMaturity=120 / MinConfirmations=6;
  testnet/regtest inherit 120, override MinConfirmations=1
- PalladiumNetworks: align NBitcoin Consensus.CoinbaseMaturity to 120
- CachedUtxo: add IsCoinbase flag (ElectrumX listunspent does not expose
  this; derived from Transaction.IsCoinBase on the raw tx)
- WalletSynchronizer: set IsCoinbase during local UTXO reconstruction
- TransactionFactory.Build: add tipHeight param; filter by threshold;
  error reports reason (immature coinbase with X/121 counter, under-
  confirmed regulars with best-seen count, mempool amounts)
- All Build() call sites updated (App, Donate, CLI)
- Two new tests: coinbase maturity boundary (119→220 tip) and mainnet
  min-conf boundary (5→6 confirmations)

src/App/ViewModels/MainWindowViewModel.Donate.cs

@@ -49,7 +49,7 @@ public partial class MainWindowViewModel
 
             _pendingDonate = new TransactionFactory(_account).Build(
                 _doc.Cache.Utxos, _lastTransactions, destination, amount, feeRate,
-                _doc.Cache.NextChangeIndex, sendAll: false);
+                _doc.Cache.NextChangeIndex, _doc.Cache.TipHeight, sendAll: false);
 
             DonatePreview = $"txid {_pendingDonate.Txid[..16]}… · " +
                 $"fee {Fmt(_pendingDonate.Fee.Satoshi)} " +

src/App/ViewModels/MainWindowViewModel.Send.cs

@@ -73,7 +73,7 @@ public partial class MainWindowViewModel
 
             _pendingSend = new TransactionFactory(_account).Build(
                 _doc.Cache.Utxos, _lastTransactions, destination, amount, feeRate,
-                _doc.Cache.NextChangeIndex, SendAll);
+                _doc.Cache.NextChangeIndex, _doc.Cache.TipHeight, SendAll);
 
             SendPreview = $"txid {_pendingSend.Txid[..16]}… · " +
                 $"fee {Fmt(_pendingSend.Fee.Satoshi)} " +

src/Cli/Program.cs

@@ -193,7 +193,7 @@ static async Task<int> Send(string[] o)
 
     var built = new TransactionFactory(account).Build(
         doc.Cache.Utxos, transactions, destination, amount, feeRate,
-        doc.Cache.NextChangeIndex, sendAll);
+        doc.Cache.NextChangeIndex, doc.Cache.TipHeight, sendAll);
 
     Console.WriteLine($"txid:  {built.Txid}");
     Console.WriteLine($"fee:   {CoinAmount.Format(built.Fee.Satoshi, account.Profile.CoinUnit)} " +

src/Core/Chain/ChainProfile.cs

@@ -126,6 +126,12 @@ public sealed record ChainProfile
     /// <summary>Target block time in seconds (LWMA v2: 120s).</summary>
     public required int BlockTimeSeconds { get; init; }
 
+    /// <summary>Blocks a coinbase output must wait before it can be spent.</summary>
+    public required int CoinbaseMaturity { get; init; }
+
+    /// <summary>Minimum confirmations required for a regular UTXO to be spendable.</summary>
+    public required int MinConfirmations { get; init; }
+
     /// <summary>BIP32/SLIP-132 headers for each script type.</summary>
     public required IReadOnlyDictionary<ScriptKind, ExtKeyHeaders> ExtKeyHeaders { get; init; }
 

src/Core/Chain/ChainProfiles.cs

@@ -26,6 +26,8 @@ public static class ChainProfiles
         ExplorerUrl = "https://explorer.palladium-coin.com/",
         SkipPowValidation = true,
         BlockTimeSeconds = 120,
+        CoinbaseMaturity = 120,
+        MinConfirmations = 6,
         ExtKeyHeaders = new Dictionary<ScriptKind, ExtKeyHeaders>
         {
             [ScriptKind.Legacy] = new(0x0488ade4, 0x0488b21e),                // xprv / xpub
@@ -53,6 +55,7 @@ public static class ChainProfiles
     {
         Kind = NetKind.Testnet,
         NetName = "testnet",
+        MinConfirmations = 1,
         WifPrefix = 0xff,
         AddrP2pkh = 127,
         AddrP2sh = 115,

src/Core/Chain/PalladiumNetworks.cs

@@ -114,7 +114,7 @@ public static class PalladiumNetworks
                 MajorityRejectBlockOutdated = 950,
                 MajorityWindow = 1000,
                 MinimumChainWork = uint256.Zero,
-                CoinbaseMaturity = 100,
+                CoinbaseMaturity = 120,
                 SupportSegwit = true,
                 SupportTaproot = true,
                 ConsensusFactory = new ConsensusFactory(),

src/Core/Spv/WalletSynchronizer.cs

@@ -225,6 +225,7 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
                     IsChange      = addr.IsChange,
                     AddressIndex  = addr.Index,
                     Height        = txHeights[txid],
+                    IsCoinbase    = tx.IsCoinBase,
                 });
             }
         }

src/Core/Storage/WalletDocument.cs

@@ -150,5 +150,6 @@ public sealed class CachedUtxo
     public bool IsChange { get; set; }
     public int AddressIndex { get; set; }
     public int Height { get; set; }
+    public bool IsCoinbase { get; set; }
     public bool Frozen { get; set; }
 }

src/Core/Wallet/TransactionFactory.cs

@@ -40,6 +40,7 @@ public sealed class TransactionFactory(IWalletAccount account)
     /// <param name="amountSats">Amount; ignored when <paramref name="sendAll"/> is true.</param>
     /// <param name="feeRateSatPerVByte">Fixed fee rate in sat/vByte (§6.4).</param>
     /// <param name="changeIndex">Index of the next change address (internal chain).</param>
+    /// <param name="tipHeight">Current chain tip height, used to enforce confirmation thresholds.</param>
     /// <param name="sendAll">Send all: fee subtracted from the amount (§6.1).</param>
     public BuiltTransaction Build(
         IReadOnlyList<CachedUtxo> utxos,
@@ -48,16 +49,52 @@ public sealed class TransactionFactory(IWalletAccount account)
         long amountSats,
         decimal feeRateSatPerVByte,
         int changeIndex,
+        int tipHeight,
         bool sendAll = false)
     {
-        // Only confirmed UTXOs are spent: mempool funds appear in the "pending"
-        // balance but are not spendable until confirmed.
-        var spendable = utxos.Where(u => !u.Frozen && u.Height > 0).ToList();
+        var coinbaseMaturity = account.Profile.CoinbaseMaturity;
+        var minConf = account.Profile.MinConfirmations;
+
+        // A UTXO is spendable when it has enough confirmations:
+        //   - coinbase: COINBASE_MATURITY + 1 confirmations (matches the Qt wallet: the consensus
+        //     rule is nSpendHeight - nHeight >= 120, so at the current tip a TX can be mined in
+        //     the next block when tipHeight - height + 1 >= 121)
+        //   - regular: MinConfirmations (wallet policy, no consensus rule)
+        int coinbaseThreshold = coinbaseMaturity + 1;
+        var spendable = utxos.Where(u =>
+            !u.Frozen &&
+            u.Height > 0 &&
+            (tipHeight - u.Height + 1) >= (u.IsCoinbase ? coinbaseThreshold : minConf)
+        ).ToList();
+
         if (spendable.Count == 0)
         {
-            var pending = utxos.Where(u => !u.Frozen && u.Height <= 0).Sum(u => u.ValueSats);
-            throw new WalletSpendException(pending > 0
-                ? $"No confirmed funds: {CoinAmount.Format(pending)} pending confirmation (not spendable)."
+            var reasons = new System.Text.StringBuilder();
+
+            var mempool = utxos.Where(u => !u.Frozen && u.Height <= 0).ToList();
+            if (mempool.Count > 0)
+                reasons.Append($"{mempool.Count} output(s) unconfirmed ({CoinAmount.Format(mempool.Sum(u => u.ValueSats))} in mempool). ");
+
+            var immature = utxos.Where(u =>
+                !u.Frozen && u.Height > 0 && u.IsCoinbase &&
+                (tipHeight - u.Height + 1) < coinbaseThreshold).ToList();
+            if (immature.Count > 0)
+            {
+                var best = immature.Max(u => tipHeight - u.Height + 1);
+                reasons.Append($"{immature.Count} coinbase output(s) not yet mature ({best}/{coinbaseThreshold} confirmations). ");
+            }
+
+            var underConf = utxos.Where(u =>
+                !u.Frozen && u.Height > 0 && !u.IsCoinbase &&
+                (tipHeight - u.Height + 1) < minConf).ToList();
+            if (underConf.Count > 0)
+            {
+                var best = underConf.Max(u => tipHeight - u.Height + 1);
+                reasons.Append($"{underConf.Count} output(s) need {minConf} confirmations ({best} so far). ");
+            }
+
+            throw new WalletSpendException(reasons.Length > 0
+                ? $"No spendable UTXOs: {reasons.ToString().TrimEnd()}"
                 : "No spendable UTXOs selected.");
         }