Capnproto v2 support #180

1. 
   ryanofsky commented at 2:12 pm on June 3, 2025: collaborator

   A Cap’n Proto v2 branch was announced https://capnproto.org/news/#whats-planned-for-20 and it would be good to make sure libmultiprocess is compatible with it. @fanquake also flagged an issue in https://github.com/bitcoin/bitcoin/issues/32669 with the v2 branch requiring GCC 14.2 or 15.1 that could affect us.

   Useful steps to take would be to:

   • Test building libmultiprocess & bitcoin with ENABLE_IPC=ON with the capnproto v2 branch.
   • If there are any incompatibilities, fix them, maybe using the CAPNP_VERSION macro if needed to handle special cases.
   • Add a CI job to the libmultiprocess repository to ensure compatibility going forward.

Contributors
ryanofsky