node: online mining can accept a foreign coinbase (--mine_online_foreign, --mine_online_reserve)

[profinch]

What

Lets a mining pool pay its miners in the block itself, with coinbase outputs the miners created and signed with their own keys, so the pool never holds their coins. It is the coinbase half of a P2Pool for Beam.

Beam is Mimblewimble: whoever creates an output can spend it, so the block builder cannot make outputs for the miners. The miners make them in advance, as pairs: a coinbase output with a public value plus a kernel whose excess is minus the output's blinding factor. Each pair balances by itself, needs no offset and no co-signer, and is made with the miner key and tagged with the owner key exactly like the coinbase of a node mining for that wallet, so the wallet finds it through its ordinary UTXO events. The pool puts the pairs for what each miner is owed into the coinbase, plus one output of its own for the rest (fee, transaction fees, rounding). Consensus already allows any number of coinbase outputs and kernels as long as they add up to the emission plus the fees (since HF6).

The node side

Online mining already lets the owner wallet finalize the block. The only thing in the way is the check at finalization that every coinbase output is recognized by the node's owner key, and the block space reserved for a single coinbase output. Two options, both off by default, so nothing changes unless they are set:

Option	Effect
--mine_online_foreign=1	Skip the "outputs correspond to our viewer's Kdf" check at finalization. If the finalization then fails in the chain context (a kernel already visible, a block too large), drop the finalizer and keep mining on the node's own miner key, instead of stalling.
--mine_online_reserve=<bytes>	Block space kept free for the coinbase when a block is assembled for finalization, on top of the single output and kernel reserved today.

41 lines over 7 files. Consensus is untouched: a node without the options accepts the blocks.

The second commit is independent and only for test chains: with --consensus=FakePoW the node left mining off whatever the options said; now --mining_threads and --pow_solve_time apply when the solve time is set explicitly, so a FakePoW node can serve its stratum server like a PoW node. Happy to drop it from this PR if you prefer.

Tested

• Two nodes in one process on a FakePoW chain with every fork active: the mining one with the options and a finalizer paying three miners, the other a stock node validating every block and serving the first miner's wallet. Both end on the same tip; every coinbase output belongs to exactly one key; the wallet finds every coin and spends one; a pair whose kernel is already in the chain makes the node drop the finalizer once and lose no block; without --mine_online_foreign the node refuses the foreign outputs.
• A private chain end to end with the stock binaries: patched beam-node, the finalizer, the pool server, the miner's tool, a CPU miner, a second stock node holding the miner wallet's owner key, and beam-wallet itself. 50 blocks; the wallet showed what the blocks paid to the groth and spent it; the fee landed in the next coinbase.
• nodetest passes.

The pool side (finalizer daemon, miner's tool, pool server) is at https://github.com/profinch/bumblebeam/tree/main/tools/coinbase, with the same patch as a file and the test harness.