Make SV2 extended channel extranonce split configurable

[average-gary]

Summary

Currently the SV2 extended channel implementation uses a fixed 6/6 extranonce split: 6-byte pool prefix (server_id[2] + channel_id[4]) and 6-byte proxy search space, within the 12-byte coinbase extranonce slot.

This works well for most cases (2^48 combinations is plenty), but a downstream proxy requesting min_extranonce_size > 6 will be rejected with unsupported-min-extranonce-size.

Proposed Enhancement

• Make the extranonce prefix length configurable (e.g., pool config or dynamic based on min_extranonce_size from proxy)
• Allow shorter pool prefixes (e.g., 2-4 bytes) to give proxies more search space when requested
• Consider making the total extranonce slot size configurable (currently hard-coded at 12 bytes via EXTRANONCE1_SIZE + EXTRANONCE2_SIZE)

Context

• SV1 uses a hard-coded 4/8 split (same 12-byte slot)
• The 12-byte slot is baked into build_coinbase_transaction() via EXTRANONCE_SEPARATOR
• Extended channel extranonce structure: coinbase_tx_prefix + extranonce_prefix + proxy_extranonce + coinbase_tx_suffix
• Related: Implement Extended Mining Channel support #8 (Extended Mining Channels), Implement SubmitSharesExtended handler #14 (SubmitSharesExtended)

Priority

Low — the fixed 6/6 split covers all realistic use cases. This is a future improvement for spec completeness.

[average-gary]

Worth noting: upstream SRI is working on a proper ExtranonceAllocator in channels_sv2 — see stratum-mining/stratum#2098 and the companion stratum-mining/sv2-apps#310. It introduces a unified, configurable allocator for extranonce prefixes (both standard and extended channels) using a bitmap for channel ID tracking.

When those land, we should evaluate adopting ExtranonceAllocator rather than building our own configurable split from scratch.