Add Windows GUI executable subsystem support

[ctate]

Summary

Adds first-class Windows executable subsystem selection through the CLI and compiler API. Windows executable builds and scriptc run accept --subsystem=console or --subsystem=windows; console remains the default, while windows links a GUI PE executable without changing scriptc’s ordinary main entry point. Invalid combinations are rejected for non-executable output and non-Windows targets.

The linker, executable caches, local artifact identities, and routed cache keys now preserve separate console and GUI variants. Windows-safe artifact and metadata replacement also allows a cached executable or generated artifact at the same path to be refreshed when the selected subsystem changes. The supported behavior is documented for the CLI and Windows cross-compilation workflow, with Windows regression coverage included.

Closes #259

[vercel]

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Project	Deployment	Actions	Updated (UTC)
scriptc	Ready	Preview, v0	Aug 31, 2026 7:59pm

[vercel]

Suggested change

	await rename(tmp, stampPath).catch(async () => {
	// POSIX rename replaces a previous same-output stamp, but Windows does
	// not. A console build followed by a GUI build deliberately has a new
	// identity at the same output path; failing to replace this metadata
	// suppresses onArtifactReady, leaving the GUI executable unavailable to
	// the bootstrap's routed cache on its next invocation.
	await rename(tmp, stampPath).catch(async (error) => {
	// POSIX rename replaces a previous same-output stamp, but Windows does
	// not. A console build followed by a GUI build deliberately has a new
	// identity at the same output path; failing to replace this metadata
	// suppresses onArtifactReady, leaving the GUI executable unavailable to
	// the bootstrap's routed cache on its next invocation. On POSIX a rename
	// failure is a genuine error (EACCES, ENOSPC, a concurrently removed
	// directory), so surface it rather than deleting a valid prior stamp.
	if (process.platform !== "win32") throw error;

publishLocalArtifactStamp's rename fallback runs the rm+retry on all platforms and swallows the original error, instead of only running on Windows and rethrowing genuine POSIX rename failures.