refactor: Signet fixups #19993

  41 | @@ -42,4 +42,5 @@ BIPs that are implemented by Bitcoin Core (up-to-date up to **v0.21.0**):
  42 |  * [`BIP 173`](https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki): Bech32 addresses for native Segregated Witness outputs are supported as of **v0.16.0** ([PR 11167](https://github.com/bitcoin/bitcoin/pull/11167)). Bech32 addresses are generated by default as of **v0.20.0** ([PR 16884](https://github.com/bitcoin/bitcoin/pull/16884)).
  43 |  * [`BIP 174`](https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki): RPCs to operate on Partially Signed Bitcoin Transactions (PSBT) are present as of **v0.17.0** ([PR 13557](https://github.com/bitcoin/bitcoin/pull/13557)).
  44 |  * [`BIP 176`](https://github.com/bitcoin/bips/blob/master/bip-0176.mediawiki): Bits Denomination [QT only] is supported as of **v0.16.0** ([PR 12035](https://github.com/bitcoin/bitcoin/pull/12035)).
  45 | +* [`BIP 325`](https://github.com/bitcoin/bips/blob/master/bip-0325.mediawiki): Signet test network is supported as of **v0.21.0** ([PR 18267](https://github.com/bitcoin/bitcoin/pull/18267)).

---

 334 | @@ -335,6 +335,10 @@ RPC
 335 |  Tests
 336 |  -----
 337 |  
 338 | +- The BIP 325 default signet can be enabled by the `-chain=signet` or `-signet`
 339 | +  setting. The settings `-signetchallenge` and `-signetseednode` allow
 340 | +  modifying the network.

---

  64 | @@ -65,8 +65,9 @@ static uint256 ComputeModifiedMerkleRoot(const CMutableTransaction& cb, const CB
  65 |      return ComputeMerkleRoot(std::move(leaves));
  66 |  }
  67 |  
  68 | -SignetTxs SignetTxs::Create(const CBlock& block, const CScript& challenge)
  69 | +Optional<SignetTxs> SignetTxs::Create(const CBlock& block, const CScript& challenge)
  70 |  {
  71 | +    const auto invalid = []() -> Optional<SignetTxs> { return nullopt; };

---

  34 | -    static SignetTxs Create(const CBlock& block, const CScript& challenge);
  35 | +    SignetTxs(const T1& to_spend, const T2& to_sign) : m_to_spend{to_spend}, m_to_sign{to_sign} { }
  36 |  
  37 |  public:
  38 | -    SignetTxs(const CBlock& block, const CScript& challenge) : SignetTxs(Create(block, challenge)) { }
  39 | +    static Optional<SignetTxs> Create(const CBlock& block, const CScript& challenge);

---