Add ECX eCash fork support

[bobbythelobster]

Summary

• Add a separate ecash.com ECX plugin as bitcoinecash, distinct from the existing XEC ecash plugin
• Reuse Bitcoin key/address parameters and expose ECX as a BTC fork for split flows
• Add opt-in ECX replay protection using nLockTime 499999999 with non-final sequence numbers
• Add metadata/fixture coverage and transaction signing coverage for ECX locktime behavior

Notes

• ECX is pre-launch/drynet4; final launch endpoints and parameters should be reverified before go-live.

Verification

• npm run lint
• npm run types
• npm test -- --grep "ECX|currency metadata"
• npm test (1274 passing, 8 pending)
• precommit hook: tsc + full npm test (1274 passing, 8 pending)

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