Normalize headerRewardMode to sum-pool-vout for several coin configs

[MoneroOcean]

Motivation

• Standardize RPC headerRewardMode across BTC-style coin configurations to ensure consistent reward VOUT handling for pooled payouts.
• Replace inconsistent modes (pool-address-vout and sum-vout) with the unified sum-pool-vout value.

Description

• Changed rpc.headerRewardMode to "sum-pool-vout" in lib/coins/btrm.js and lib/coins/rtm.js, preserving rewardMultiplier and other settings.
• Changed rpc.headerRewardMode to "sum-pool-vout" in lib/coins/rvn.js and lib/coins/xna.js (was "sum-vout").
• No other behavioral changes to pow, difficultyMultiplier, or rewardMultiplier values.

Testing

• Ran the project's automated test suite with npm test and the linter with npm run lint, and all tests passed.

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Codex Task