v0.2.4 - Lithos test-network-ready + solution-buffer hardening

Lithos is now test-network-only, not experimental. The Lithos stratum support
is tested end to end against a live Lithos client (v4.2.0) on the Ergo test
network: the miner connects, receives real jobs, and its shares are accepted by
the client (95 accepted / 0 rejected in the run that cleared it), not just checked
against the mock. It has not been validated on mainnet yet, so run it on
testnet only for now - the bundled lithos-quickstart.sh defaults to testnet on
purpose. See the "Lithos" section of the README for the from-scratch guide.

Hardened the search solution buffer. One search batch at a deliberately low
difficulty (exactly the Lithos-test regime) can find far more than the old
32-solution buffer held, and anything past it was silently dropped - a bug that
scales with hashrate and masquerades as flaky hardware. The buffer is now 256
(still 10 KB) and the miner warns when a batch overflows instead of losing shares
invisibly.

No dev fee. make test green on both backends.