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/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,