Windows support for skit

[leogdion]

skit currently doesn't run on Windows. The library is largely platform-agnostic, but several gaps need closing before the executable is usable.

Known gaps

• Sources/SyntaxKit/Execution/String+DylibFilename.swift: add an os(Windows) case returning .dll (it currently falls through to .dylib, which is wrong on Windows).
• Sources/SyntaxKit/Execution/OutputCache.swift: add a Windows cache-root branch (%LOCALAPPDATA%\SyntaxKit\cache via FOLDERID_LocalAppData or the env var fallback).
• Build-bundle flow: bash scripts don't run natively on Windows. Options:
  • PowerShell sibling (Scripts/build-skit.ps1).
  • Move bundle creation into a Swift target that runs on every platform (preferred long-term; less duplication).
• DLL search-path strategy: Windows uses the binary's directory as a default search path, so the macOS-style lib/ subdirectory layout may need SetDllDirectoryW or a flat layout. Verify with a smoke test.
• TerminationStatus+ExitCode.swift:41 already guards .signaled with #if !os(Windows) — confirm the Windows path still returns a meaningful exit code on abnormal termination.
• Help text and diagnostics still say "libSyntaxKit.dylib" — make them platform-aware.
• CI: add a Windows job that runs a built bundle end-to-end (currently only `swift build`/`swift test`).

Dependencies

• `swift-subprocess` Windows backend is less battle-tested than macOS/Linux; expect to hit issues and contribute upstream.

Out of scope

• Native Windows installer (MSI/WinGet).
• Cross-compiling from macOS/Linux (deferred until the native build is proven).