Chained mod target overlays

- Mod targets list an ordered overlay chain; later overlays replace earlier ones and earlier source replacements apply to inherited files
- Forge and NeoForge metadata take their loader version range from the target
- The Fabric permissions library is bundled only for targets that declare it
- Dropped the 1.21.1 mods.toml copies that only differed in the loader range

gradle/mod-target.gradle

@@ -8,6 +8,9 @@ Map<String, Object> target = targets.get(targetMinecraft)
 if (target == null) {
     throw new GradleException("Unsupported Iris mod target ${targetMinecraft}; choose ${targets.keySet().join(', ')}")
 }
+targets.each { String minecraft, Map<String, Object> candidate ->
+    candidate.put('overlays', candidate.get('overlays') ?: [minecraft])
+}
 ext.irisModTarget = target
 ext.irisModTargets = targets
 if (project.name.startsWith('iris-')) {