Implement Electrum Protocol v1.6#190

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tnull wants to merge 7 commits intobitcoindevkit:masterfrom

tnull:2026-01-protocol-v1.6

Commits on Jan 6, 2026

feat: Add `server_version` method for protocol version negotiation
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feat: Add `mempool_get_info` method for v1.6 mempool fee information
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feat: Add optional `EstimationMode` parameter to `estimate_fee`
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feat: Automatically negotiate protocol version on connect
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feat: Support v1.6 `block_headers` response format with backwards compatibility
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feat: Add `transaction_broadcast_package` method for package relay
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docs: Mark `relay_fee` as deprecated in protocol v1.6+
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