Implement SubmitSharesExtended handler

[average-gary]

Summary

Handle SubmitSharesExtended messages from extended channel proxies, validate proof-of-work, and respond with success/error.

Context

Extended channel submissions include the extranonce chosen by the proxy, in addition to the standard fields. The ExtendedChannel::validate_share() method validates the full share including the proxy-chosen extranonce against the allocated prefix and size.

Tasks

• Route SubmitSharesExtended to the correct ExtendedChannel by channel_id
• Call extended_channel.validate_share(msg)
• Handle ShareValidationResult variants (same as standard, but with extended extranonce context):
  • Valid(hash) -> SubmitShares.Success
  • BlockFound(hash, template_id, coinbase) -> trigger block assembly
  • Error variants -> SubmitShares.Error
• Validate that submitted extranonce falls within the allocated prefix + size range
• Batch success responses for efficiency
• Track share counts per extended channel

Acceptance Criteria

• Valid extended shares with correct extranonce are accepted
• Extranonce outside allocated range is rejected
• Block-found triggers block submission
• Unit tests: valid extended share, block found, extranonce validation, invalid extranonce

Dependencies

• Implement Extended Mining Channel support #8 (Extended channels)
• Implement SV2 job distribution to channels #11 (Job distribution)
• Implement SubmitSharesStandard handler #13 (Standard shares handler -- shared response infrastructure)

[average-gary]

SubmitSharesExtended handler fully implemented and tested. Includes validate_extended_share (coinbase reconstruction, merkle path walk, PoW check), emit_extended_share (shared emission pipeline), and wrong-extranonce-size rejection. Commits: p2pool-v2 73473d8 + bcc6f5e.