DashParams extension

Dash chain parameters.

DIP-2 special transactions (ProRegTx, ProUpServTx, ProUpRegTx, ProUpRevTx, cbTx, QcTx) are supported at the wire level: Dash packs the special-tx type in the high 16 bits of the version word (nVersion | (nType << 16)), so a Dash special tx parses to an ExTx with txType exposing the nType and the trailing vExtraPayload preserved unchanged through round-trips, txid, and the legacy sighash preimage. This is the same path Firo uses; see test/tx/dash_special_tx_vectors_test.dart. Structured DIP-2 payload decoding and BLS operator-key handling are out of scope (the payload is an opaque blob).