feat(wallet): surface immature balance separately from confirmed

Extract confirmation-threshold logic into UtxoSpendability (shared by
TransactionFactory and WalletSynchronizer) and use it to compute
ImmatureSats, so coinbase/under-confirmed amounts are shown separately
from spendable balance in the GUI and CLI instead of being lumped into
"confirmed".

src/Core/Wallet/UtxoSpendability.cs

@@ -0,0 +1,24 @@
+using PalladiumWallet.Core.Chain;
+using PalladiumWallet.Core.Storage;
+
+namespace PalladiumWallet.Core.Wallet;
+
+/// <summary>
+/// Confirmation-threshold rules shared between coin selection (<see cref="TransactionFactory"/>)
+/// and balance reporting: a coinbase output needs COINBASE_MATURITY + 1 confirmations
+/// (consensus rule nSpendHeight - nHeight >= 120, plus one block of safety margin, matching
+/// the Qt wallet); a regular output needs <see cref="ChainProfile.MinConfirmations"/> (wallet
+/// policy, no consensus rule).
+/// </summary>
+public static class UtxoSpendability
+{
+    public static int RequiredConfirmations(this CachedUtxo utxo, ChainProfile profile) =>
+        utxo.IsCoinbase ? profile.CoinbaseMaturity + 1 : profile.MinConfirmations;
+
+    public static int Confirmations(this CachedUtxo utxo, int tipHeight) =>
+        utxo.Height <= 0 ? 0 : tipHeight - utxo.Height + 1;
+
+    /// <summary>True when the UTXO has met its confirmation threshold and is not frozen.</summary>
+    public static bool IsSpendable(this CachedUtxo utxo, ChainProfile profile, int tipHeight) =>
+        !utxo.Frozen && utxo.Height > 0 && utxo.Confirmations(tipHeight) >= utxo.RequiredConfirmations(profile);
+}