internal/rosa: self-host stage0 tarball

This replaces gentoo stage3 tarballs.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/all.go

@@ -83,6 +83,10 @@ const (
 	// stages only. This preset and its direct output must never be exposed.
 	gcc
 
+	// Stage0 is a tarball containing all compile-time dependencies of artifacts
+	// part of the [Std] toolchain.
+	Stage0
+
 	// _presetEnd is the total number of presets and does not denote a preset.
 	_presetEnd
 )