feat: add SEWASStarPU distributed execution engine and switch Linux CI to it

[jfonseca-aneo]

Summary

Adds SEWASStarPU, a new distributed execution engine built on StarPU-MPI, and switches Linux CI's distributed backend from PaRSEC to it via SEWAS_WITH_STARPU.

This is motivated by a real, bisected upstream PaRSEC defect: a race in PaRSEC's remote-dependency engine corrupts or hangs cross-rank typed halo exchange on every release since parsec-3.0.2012-rc1. SeWaS's current pin to the last pre-regression commit is a mitigation, not a fix.

Before writing any production code, we validated the migration path with a standalone de-risking spike (kept local, not part of this PR — see note below). Key result: StarPU-MPI's high-level task_insert API reproduces a comparable deadlock, but its lower-level explicit isend_detached/irecv_detached API does not, across 124 trials at loads well beyond anything that broke PaRSEC. SEWASStarPU is built on the explicit send/recv API accordingly.

Changes

• include/SEWASStarPU.hxx, src/SEWASStarPU.cxx — new StarPU-MPI-based execution engine, wired into LinearSeismicWaveModel::propagate() and ExecutionContext alongside the existing PaRSEC/sequential dispatch. Task priorities reuse the existing MinimumCommunicationPriorityEvaluator.
• CMakeLists.txt, cmake/resources/starpu/CMakeLists.txt — new SEWAS_WITH_STARPU option (mutually exclusive with SEWAS_WITH_PARSEC); StarPU is located via pkg-config and, when missing, auto-downloaded/built the same way PaRSEC/ADIOS2 already are (gated by DOWNLOAD_MISSING_DEPS).
• .github/workflows/Linux.yml — Linux CI now configures with -DSEWAS_WITH_STARPU=ON instead of -DSEWAS_WITH_PARSEC=ON, and drops the flex/bison install step (only needed for PaRSEC's parsec-ptgpp). PaRSEC itself is untouched and still buildable via SEWAS_WITH_PARSEC=ON, just no longer exercised by CI.
• README.md — updated to describe StarPU as CI's distributed backend and PaRSEC as the (still supported) alternative.
• Misc: bump ADIOS2 pin, drop a now-redundant vcpkg arm64-linux patch, bump actions/checkout to v4, README rewrite for the vcpkg-based build workflow, PaRSEC 4.0 arena/datatype API port, googletest pin.

Note

The de-risking spike and its feasibility write-up were kept out of version control (local reference only) since they're throwaway investigation artifacts, not something the repo needs to carry going forward.

Test plan

• Linux CI passes with the new -DSEWAS_WITH_STARPU=ON configuration (ctest, including TestC/TestCSplitX/Y/Z) — note: CI has apparently never executed on this repo (0 recorded workflow runs), so this may need the Actions setting sorted out first
• -DSEWAS_WITH_PARSEC=ON still builds and passes locally (no longer covered by CI, but not removed)