Refactor /common and beta buffer routines for future particle inclusion

[joshgillis]

Description

The beta buffer communication routines were generalized to support both bubble and particle data. Previously, these routines were hard-coded to use beta_vars, which defines the communication mapping for bubble variables. Particle communication will require a different mapping (vars_send). Instead of duplicating the communication routines for each case, a generic communication variable list (vars_comm) is now passed through the call chain. Bubble routines pass beta_vars, while particle routines will pass vars_send, allowing the same communication infrastructure to be reused for both implementations.

Type of change (delete unused ones)

• Refactor

Testing

How did you test your changes?

Test suite ran on personal machine cpu, and tuolumne gpu

Checklist

Check these like this [x] to indicate which of the below applies.

• I added or updated tests for new behavior
• I updated documentation if user-facing behavior changed

See the developer guide for full coding standards.

GPU changes (expand if you modified src/simulation/)

• GPU results match CPU results
• Tested on NVIDIA GPU or AMD GPU

AI code reviews

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[github-actions]

Claude Code Review

Head SHA: df0a969

Files changed:

• 4
• src/common/m_boundary_common.fpp
• src/common/m_boundary_primitives.fpp
• src/common/m_mpi_common.fpp
• src/simulation/m_bubbles_EL.fpp

Findings:

• src/common/m_boundary_primitives.fpp: s_beta_extrapolation gains a new mandatory vars_comm dummy argument (and its body now indexes via idx = vars_comm(i) instead of beta_vars(i)), but no hunk in this diff updates any call site to pass the new argument — s_beta_periodic and s_beta_reflective had their call sites updated in m_boundary_common.fpp, s_beta_extrapolation did not. If this routine is invoked anywhere with the old 6-argument form, the build breaks; please confirm every caller was updated.
• src/common/m_boundary_common.fpp, src/common/m_boundary_primitives.fpp, src/common/m_mpi_common.fpp: the new vars_comm parameter is threaded through s_populate_beta_buffers → s_populate_beta_bc_direction → s_beta_periodic/s_beta_reflective → s_mpi_reduce_beta_variables_buffers, but both call sites in m_bubbles_EL.fpp pass the same beta_vars array that was previously hardcoded inside these routines. As written, this is generalization with no current caller variation — per CLAUDE.md's guidance against "abstractions for one call site," consider whether this parameter is actually needed yet or should be added alongside the code that first varies it.

[joshgillis]

I'm not sure what the review means about the updated call site for s_beta_extrapolation, I'm actually not even sure if where that subroutine is called. I can remove the changes there if that subroutine is not in use.

As for the PR as a whole, if it is preferred that this structure is not added until particles are functional I can wait and make other adjustments.

[wilfonba]

I believe s_beta_extrapolation can just be removed. It must have slipped through at some point. Extrapolation boundary conditions for beta don't make any physical sense.