Build 1.21.5 with Stonecutter from the shared src/

1.21.5 is the first version before the 1.21.6 GUI rewrite, which moved
GUI drawing from vertex consumers and a MatrixStack to render states and
a 2D Matrix3x2fStack.

- RenderScopeImpl and EntityDrawerImpl have two whole implementations,
  one per side of 1.21.6 (a single >=1.21.6 / else split), instead of
  conditionals scattered through them. Callers use RenderScopeImpl(context)
  in every version; 1.21.5's did the same thing through its
  (matrices, vertexConsumers) constructor.
- MatrixStackMixin (MatrixStack as a MatrixStackRef) is real before 1.21.6
  and empty from 1.21.6, registered everywhere like NametagsMixin.
- DrawContextAccessor exposes the vertex consumers before 1.21.6 and the
  GUI render state from it.
- Smaller changes: fog (BackgroundRenderer vs FogRenderer), the end
  portal texture arguments (ShaderUtils, now a flat three-way chain), the
  status effect sprite manager, and getIconId only from 1.21.6.

Features 1.21.6+ lacks stay on 1.21.5, with a TODO: the title panorama
behind Saturn screens and the background blur (screen.backgroundBlur,
see #23).

The 1.21.5 jar differs from the old one only in the helpers from earlier
steps, the RenderScopeImpl(context) calls, the empty SaturnRenderState
and unused Matrix3x2fStackRef, and 1.21.5's dead NameTagMixin (never
registered) being gone. 1.21.6-1.21.11 only gain the empty
MatrixStackMixin and its impl.mixins.json entry.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

settings.gradle.kts

@@ -24,7 +24,7 @@ include("common")
 
 // Versions built by Stonecutter from the shared src/ (see stonecutter.gradle.kts). Each one's
 // versions/<v>/gradle.properties holds its Minecraft, Yarn and Fabric API versions.
-val stonecutterVersions = listOf("1.21.6", "1.21.7", "1.21.8", "1.21.9", "1.21.10", "1.21.11")
+val stonecutterVersions = listOf("1.21.5", "1.21.6", "1.21.7", "1.21.8", "1.21.9", "1.21.10", "1.21.11")
 
 stonecutter {
     create(rootProject) {