feat: Add Windows platform support (MSVC and MinGW)

[quinnjr]

Summary

Adds Windows platform support to PluMA's C++ core (MSVC and MinGW toolchains) and to the SConstruct build configuration. The implementation is non-invasive on Unix: existing Linux / macOS builds compile and link identically, and all current upstream contributions (Rust loader, parallel scheduler, plugin venv, build fixes from PRs #7, #8, #10, #11, #13) are preserved unchanged.

What it adds

C++ core

• src/platform.h — single header that abstracts the three POSIX primitives PluMA's core relies on but Windows lacks or names differently:
  • dlopen / dlsym / dlclose / dlerror → LoadLibrary / GetProcAddress / FreeLibrary / Windows error formatting on _WIN32
  • glob / glob_t → a Windows shim built on FindFirstFile / FindNextFile
  • POSIX path separators / unistd.h / pthread includes → conditional wrappers
• src/main.cxx, src/languages/Compiled.cxx, src/languages/Language.{cxx,h} — use src/platform.h instead of including <dlfcn.h> / <glob.h> / <unistd.h> directly.

Build system

• build_config.py — adds is_msvc and is_mingw toolchain detection (alongside the is_windows / is_darwin / is_linux / is_alpine flags upstream master already provides) and platform-specific filename conventions (shared_lib_ext, shared_lib_prefix, executable_ext, path_separator). Windows overrides lib_search_path and include_search_path because the /lib + /usr/lib + /usr/local/lib convention doesn't apply.
• SConstruct:
  • get_platform_config() picks up Windows / MSVC / MinGW branches that set PLUMA_PLATFORM_WINDOWS, WIN32_LEAN_AND_MEAN, and NOMINMAX defines (so src/platform.h selects the LoadLibrary / GetProcAddress code path). MinGW additionally gets -Wl,--export-all-symbols and psapi linkage.
  • create_base_environment() splits CCFLAGS / CXXFLAGS into an MSVC branch (/EHsc /utf-8 /O2) and a gcc-style branch (-fpermissive -fPIC -O2).
• .github/workflows/windows-build.yml — Windows CI smoke test using windows-latest runners with both Visual Studio and MinGW.

What it does not do (deferred)

• Rust plugin loading on Windows. src/languages/Rust.cxx (added by PR Add Rust language support for PluMA plugins #8) still uses POSIX dlopen / dlsym directly rather than the src/platform.h abstraction. Threading the abstraction through the Rust loader is a small but separable follow-up; the main pluma binary still builds on Windows without it, and Rust plugin support remains Unix-only for now.
• Python / Perl / R embedded interpreters on Windows. Each of those embeds an upstream interpreter that has its own Windows story (Python is fine; Perl on Windows needs a strawberry-perl-style build; R on Windows requires Rtools alignment). Out of scope for this PR.
• Java plugin loading on Windows. JNI itself is portable; the plugin-discovery glob path needs the same src/platform.h treatment as the Rust loader before Windows-side Java plugins can be loaded.

Compatibility

• Linux / macOS: unchanged. Verified on Arch Linux (clang 21 / R 4.6 / perl 5.42 / Python 3.14 / OpenJDK 17) that scons --with-rust still produces a working pluma binary with dlsym imported (i.e. HAVE_RUST wiring from PR build(scons): fix master build and wire --with-rust through to HAVE_RUST #13 intact).
• Windows: build target is the C++ core. The accompanying CI workflow exercises the SConstruct branches on windows-latest with both MSVC and MinGW.

Notes on the merge

The branch was originally cut from 819d31a (pre-PR-#7), before the SConstruct refactor and the subsequent Rust / parallel / venv / build-fix series merged. To bring the branch onto current master without rewriting upstream history, I merged upstream/master in and resolved by taking master's refactored versions of SConstruct / build_config.py / build_support.py and re-applying the Windows-specific additions on top. The single merge commit (9563464) captures that resolution; the two original Windows commits (5f1d6e8 for the platform layer, b50ddad for CI) are unchanged.

Test plan

• scons --with-rust on Linux produces a pluma binary with dlsym imported (HAVE_RUST wired)
• nm pluma | grep -c dlsym returns ≥ 1
• Existing Catch2 tests on master continue to build via the CMake test harness
• Windows CI smoke test passes on the PR's GitHub Actions run
• MSVC and MinGW builds both produce a pluma.exe

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