ENH: Add RPC support

[HaoZeke]

The design is still being threshed out.

meson setup bbdir -Dwith_rpc=True
meson compile -C bbdir
./bbdir/CppCore/rgpot/rpc/potserv 125697
python CppCore/rgpot/rpc/pyclient.py localhost:125697

[HaoZeke]

Note that, thankfully, there is no real difference between working with this "simpler interface":

@0xbd1f89fa17369103;

# Design kanged from eOn v4 C-style structs
# TODO(rg): Should be more object oriented

struct ForceInput {
 natm   @0 :Int32; # TODO(rg): Do we really need this..
 pos    @1 :List(Float64);
 atmnrs @2 :List(Int32);
 box    @3 :List(Float64);
}

struct PotentialResult {
  energy @0: Float64;
  forces @1: List(Float64);
}

interface CuH2Pot {
  calculate @0 (fip :ForceInput)
    -> (result :PotentialResult);
}

Or this redundant one:

struct Position {
  x @0 :Float64;
  y @1 :Float64;
  z @2 :Float64;
}

struct AtomMatrixRPC {
  positions @0 :List(Position);
}

struct BoxMatrix {
  box @0: List(Vector);
}

struct Vector {
  x @0: Float64;
  y @1: Float64;
  z @2: Float64;
}

struct AtomTypes {
  atomTypes @0: List(Int32);
}

struct PotentialResult {
  energy @0: Float64;
  forces @1: List(ForceVector);
}

struct ForceVector {
  x @0: Float64;
  y @1: Float64;
  z @2: Float64;
}

interface CuH2Pot {
  calculate @0 (positions :AtomMatrixRPC, atomTypes :AtomTypes, boxMatrix :BoxMatrix)
    -> (result :PotentialResult);
}

Which is essentially the design in 0e3d99c and 1dd36ad.

Basically there is no time difference for running two servers.

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
python oclient.py localhost:12345	108.8 ± 12.7	98.9	143.3	1.00
python pyclient.py localhost:12346	109.4 ± 18.3	99.0	182.8	1.01 ± 0.21

Where oclient is the ForceInput variant, and pyclient is the redundant form.

Essentially this seems to suggest that more nested structures are OK, which is better (needs testing of course for buffer size)

[Copilot]

Pull Request Overview

This PR introduces RPC (Remote Procedure Call) support to the project using Cap'n Proto. The implementation allows for remote calculation of potential energies and forces for the CuH2 potential through a client-server architecture.

• Adds Cap'n Proto RPC server and Python client implementation for remote potential calculations
• Updates C++ standard to C++20 and adds necessary dependencies (capnproto, pycapnp, etc.)
• Includes build system integration with conditional RPC compilation

Reviewed Changes

Copilot reviewed 12 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pixi.toml	Adds Cap'n Proto and Python dependencies for RPC support
meson_options.txt	Adds with_rpc build option to conditionally enable RPC features
meson.build	Updates C++ standard from C++17 to C++20 for RPC compatibility
CppCore/rgpot/rpc/server.cpp	Implements Cap'n Proto RPC server for CuH2 potential calculations
CppCore/rgpot/rpc/pyclient.py	Python client for connecting to and using the RPC server
CppCore/rgpot/rpc/meson.build	Build configuration for RPC server and library
CppCore/rgpot/rpc/Potentials.capnp	Cap'n Proto schema definition for potential calculation interface
CppCore/rgpot/LennardJones/meson.build	Code formatting improvements
CppCore/rgpot/CuH2/meson.build	Code formatting improvements
CppCore/meson.build	Integrates RPC subdirectory when option is enabled
.pre-commit-config.yaml	Adds meson formatting to pre-commit hooks
.github/workflows/build_test.yml	Renames job from buildmamba to buildpixi

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