Independent per-chain target checking and AuxPoW proof submission

[Boblepointu]

Summary

When a miner submits a share, compare the PoW hash against each configured external chain's difficulty target independently. A share can solve LTC without meeting DOGE difficulty, or vice versa — standard multi-chain merged mining behavior.

Depends on PR #104 (Multi-chain merge-mine manager).

What's included

Component	Files	Description
ShareValidationResult	stratumsubmit.{h,cpp}	Replaces simple ShareResult enum; flags dogeBlockFound, shareAccepted, plus auxChainsSolved vector
ValidateShareEx	stratumsubmit.cpp	Computes PoW hash once, tests against DOGE target + each external chain's nBits independently
NBitsToTarget	stratumsubmit.cpp	Compact nBits → arith_uint256 conversion
BuildAuxPowForChain	stratumsubmit.cpp	Constructs serialized CAuxPow proof for a specific solved external chain
MergeMineCommitment	stratumjob.h	Struct holding aux tree root, tree size, nonce embedded in the parent coinbase
AuxChainTarget	stratumjob.h	Per-chain nBits snapshot stored in each StratumJob
CreateJob	stratumjob.cpp	Injects fabe6d6d merge-mine commitment into coinbase scriptSig
HandleSubmit	stratum.cpp	Iterates auxChainsSolved and submits AuxPoW proofs per chain
OnExternalWorkUpdate	stratum.{h,cpp}	Triggers new job broadcast when external chain work changes
Init wiring	init.cpp	GlobalAuxManager lifecycle, MergeMineManager start/stop, work callback, startup banner

How it works

Miner submits share
    │
    ├── hash < DOGE target? → submitblock to doged
    ├── hash < LTC target?  → build CAuxPow, submit to LTC via RPC
    ├── hash < BELLS target? → build CAuxPow, submit to BELLS via RPC
    └── hash ≥ worker diff?  → accept share for hashrate accounting

Each chain is checked independently — a single share can solve multiple chains simultaneously.

Test plan

• ninja check-stratum — all unit tests pass
• Integration: mine against doged with 2 external chains at low regtest difficulty, verify blocks appear on all chains
• Verify startup banner and summary log show configured chains