AuxPoW merge-mining RPCs: createauxblock / submitauxblock

[Boblepointu]

Summary

Add createauxblock and submitauxblock JSON-RPC methods that enable external miners to request and submit Auxiliary Proof-of-Work blocks, the standard interface for Dogecoin merge-mining.

Depends on PR #102 (Stratum v1 core).

What's included

Component	Files	Description
AuxPoW manager	stratumaux.{h,cpp}	Work cache with FIFO eviction, CreateWork / GetWork / RemoveWork, commitment building
RPC methods	rpc/mining.cpp	createauxblock(coinbase_addr) → returns hash + target; submitauxblock(hash, auxpow_hex) → validates and submits
Unit tests	stratumaux_tests.cpp	AuxPoW block construction, validation, cache eviction
Functional tests	stratum_auxpow.py, stratum_mining.py	End-to-end: create aux block, build proof, submit; share submission flow
Build	CMakeLists.txt	Add stratumaux module to server library and test suite

Usage

# Request work
doged createauxblock DYourAddress
# → {"hash": "...", "chainid": 1, "previousblockhash": "...", "target": "..."}

# Submit solved block
doged submitauxblock <hash> <serialized_auxpow>
# → true

Test plan

• ninja check-stratum — stratumaux test suite passes
• test/functional/stratum_auxpow.py — e2e create + submit cycle
• test/functional/stratum_mining.py — share submission flow