ZEC Harness: Upgrade to NU6.1 (zcashd v6.11.0)

[dev-warrior777]

This PR updates the ZEC regtest harness to support NU6.1, enabling testing of the DEX wallet integration with the latest Zcash protocol.

Key Changes

• NU6.1 activation: Added nuparams=c8e71055:5 and nuparams=4dec4df0:7 (ZIP 253/255)
• Transparent addresses: Switched from z_getaddressforaccount → getnewaddress (deprecated in v6+)
• Regtest compatibility: Added -allowdeprecated=getnewaddress
• Robustness: fixed sendtoaddress spacing
• All nodes activated: NU6.1 params now on alpha, beta, delta, gamma

Why

• zcashd v6.11.0 defaults to unified addresses and account 0 behavior
• DEX harness uses transparent UTXOs → must use getnewaddress
• Without these, go test fails with "account not found" or empty UTXOs

Tested

./harness.sh
./beta getbalance   # → 74.00000000