Bump supported Bitcoin P2P protocol version past 70015

[fpelliccioni]

Current state

`kth::domain::message::version::level::maximum` is pinned to `bip152_fix = 70015` in `src/domain/include/kth/domain/message/version.hpp`. This is the highest protocol version we advertise on handshake and the one we pass to `to_data` / `from_data` when (de)serializing P2P messages.

70015 is pre-BCHN-fork. BCHN has bumped the wire version multiple times since, and ABC / BCHN / Knots have different ceilings today. We should at least catch up to the latest version BCHN supports.

What we need to figure out

• What is BCHN's current `PROTOCOL_VERSION` (and what features each bump brought in — likely things like `send_headers` ordering, `xversion`/`extversion`, service-bit changes). Start here: https://github.com/bitcoin-cash-node/bitcoin-cash-node/blob/master/src/version.h.
• Same check for BCHA / Bitcoin ABC if we still want to federate with them.
• Decide whether we follow BCHN 1:1 or stay one step behind for compatibility.

What needs to change once we pick a target

• Add the new `level` enumerators in `version.hpp` (e.g. `bchn_ = 700NN`) and update `maximum` accordingly.
• For each new message type gated by the new version (e.g. `extversion`), add/regenerate the C-API binding once the C++ side lands.
• Review every `from_data` / `to_data` that takes `uint32_t version` for behavior changes at the new level. Most are stable, but `send_compact` / `block` / `header` can shift on protocol bumps.
• Handshake logic in `kth/network`: bump the advertised version in `version_message` construction.
• Update the tests in `src/domain/test/message/` that currently use `version::level::minimum` as a sentinel — the round-trip tests need to cover the new ceiling too.

Scope note

This is a research + coordination task, not a one-commit fix. It needs a separate follow-up per message type once we know what BCHN added.