Phase 4: Windows build spike + CI

[neuromechanist]

Part of epic #147.

Scope

Windows support needs its own toolchain spike before it can be a CI job: build_amica.sh
targets a POSIX toolchain (gfortran + OpenMPI + LAPACK via apt/brew), and Windows has no
equivalent out of the box.

Approach

• Spike: MSYS2/MinGW-w64 gfortran + MS-MPI (or MPICH) + a LAPACK source (reference LAPACK build
  or OpenBLAS), or cross-compiling from a Linux host with MinGW.
• Target both Windows x86_64 and Windows arm64 if the toolchain supports arm64 (MSYS2/MinGW-w64
  arm64 Fortran+MPI support is newer and may not be viable yet).
• If arm64 native build proves infeasible, ship x86_64 only and document that it runs under
  emulation on Windows-on-ARM; do not block this phase on arm64.

Definition of Done

• Documented, scripted build path for Windows x86_64 (mirrors build_amica.sh's structure:
  override-able compiler/MPI/LAPACK vars, single build script).
• Windows arm64 build attempted; outcome (works / dropped + why) documented either way.
• Wired into the same CI release-asset job as Phase 3: CI build + publish binaries (Linux + macOS) #150 (phase 3) once a build path works.
• Manifest from Phase 1: Platform-detection + binary-fetch utility #148 updated with the new Windows entries.

[neuromechanist]

Delivered under #165 phase 2 (#167/#174): windows-x64 builds and smoke-tests green. windows-arm64 has no native Fortran toolchain (runs the x64 binary via emulation) and is tracked in #173. Closing as completed; see #147.