Rename direction-agnostic _rsx_ to _rs_ in m_riemann_solvers/m_rhs/m_viscous/m_surface_tension (NOT m_cbc)

[sbryngelson]

In the Riemann/reconstruction modules, the reshaped working arrays carry an _rsx_ suffix — e.g. qL_prim_rsx_vf, qR_prim_rsx_vf, flux_rsx_vf, flux_src_rsx_vf, flux_gsrc_rsx_vf, vel_src_rsx_vf, mom_sp_rsx_vf, Re_avg_rsx_vf, qL_rsx_vf/qR_rsx_vf, dqL_rsx_vf/dqR_rsx_vf, vSrc_rsx_vf. The x reads as "x-direction," but these buffers hold the reshaped data for whichever split direction is currently being swept (the same array is reused for the x, y, and z sweeps via norm_dir/id). The x is a historical artifact and is actively misleading. The module's own doc comments already describe these states with the direction-agnostic _rs_ form ("…given in qK_prim_rs_vf … where ds = dx, dy or dz").

Proposed: rename _rsx_ → _rs_ in the four files where it is direction-agnostic:

• src/simulation/m_riemann_solvers.fpp (453 occurrences)
• src/simulation/m_rhs.fpp (34)
• src/simulation/m_viscous.fpp (4)
• src/simulation/m_surface_tension.fpp (8)

⛔ Explicitly EXCLUDE m_cbc.fpp

src/simulation/m_cbc.fpp also contains rsx (24×), but there it is genuinely direction-specific: it is one leg of an rsx/rsy/rsz triple (q_prim_rsx_vf/rsy/rsz, F_rsx_vf/rsy/rsz, flux_rsx_vf_l/rsy/rsz), and the x/y/z is Fypp-generated via rs${XYZ}$ from #:for CBC_DIR, XYZ in [(1,'x'),(2,'y'),(3,'z')]. Renaming rsx→rs there would collide with rsy/rsz and break the build. m_cbc's directional triple belongs to the struct-consolidation work in #1440, not here.

Why it's worth it: _rsx_ is the single biggest readability tax in m_riemann_solvers — a newcomer reading the y/z sweep arms sees "x" arrays everywhere. _rs_ is the right target: it matches an existing _rs_-family convention in the tree, and the #1426 refactor's own design sketch already writes qL_prim_rs_vf / build_mixture_state(...). So this rename is aligned with #1426, not opposed.

Pure rename, no behavior change: confirmed — rsx is never a string key, namelist var, index, or IO token (outside m_cbc's Fypp generation), and there are zero references in the Python toolchain, docs, tests, or case files. Golden files compare numeric output, so a name-only change won't move them.

Sequencing (important)

This module is under active refactoring (#1426 dedup; #1440 directional dispatch via #1432, currently draft). A ~500-line rename landing concurrently would cause merge pain in the most-edited file. Recommend either:

• (a) fold this in as a "phase 0" of Refactor m_riemann_solvers: reduce duplication, improve structure, add schemes #1426 so its derived-type work moves already-clean names, or
• (b) land it as a standalone PR before Refactor m_riemann_solvers: reduce duplication, improve structure, add schemes #1426's build_mixture_state section.

Either way, sequence it ahead of #1426's structural changes, not alongside.

Gotcha

GPU_DECLARE/@:ALLOCATE/@:DEALLOCATE lines carry these names (e.g. $:GPU_DECLARE(create='[flux_rsx_vf, flux_src_rsx_vf]')) and must be renamed in lockstep. A miss is a compile error, not a silent bug. Rebuild + run the full simulation suite across the CI compilers (golden files should not move).

Related: #1426, #1440.

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_{Filed from a repo-wide code-cleanliness review; verified against master @ 40dde5e.}

Code references

• src/simulation/m_riemann_solvers.fpp:34-67 — _rsx_ array declarations (453 uses in file)
• src/simulation/m_rhs.fpp — _rsx_ (34)
• src/simulation/m_viscous.fpp — _rsx_ (4)
• src/simulation/m_surface_tension.fpp — _rsx_ (8)
• src/simulation/m_cbc.fpp — ⛔ EXCLUDE: genuine rsx/rsy/rsz triple via rs${XYZ}$