Build 1.21.8 with Stonecutter from the shared src/

Step 4 of #8, the first version before the 1.21.9 render queue.
versions/1.21.8/src is gone; 1.21.8 builds from src/ as :1.21.8.

API differences behind version comments (<1.21.9 / >=1.21.9):
- Player cosmetics: FeatureRenderer.render takes a VertexConsumerProvider
  instead of the render queue. The cloak, hat and OBJ renderers name the
  render target "output" in both, so only the parameter type and the final
  draw call differ; the OBJ face loop moved into drawGroup.
- PlayerEntityRenderer.updateRenderState's entity type, the cape texture
  accessor, GameProfile getName()/getId() vs name()/id(), fonts in
  TextMixin, EntityRenderDispatcher vs EntityRenderManager, Screen input
  events (Click/KeyInput), and KeyBinding categories and isKeyPressed.

Features 1.21.9+ lacks stay on 1.21.8, with a TODO for 1.21.9+:
- NametagsMixin moves into src/. It hooks renderLabelIfPresent with a
  VertexConsumerProvider, which 1.21.9 changed, so on 1.21.9+ its body is
  empty; it stays registered, so features.mixins.json is the same for
  every version.
- The splash screen's loading-bar border, and the real drag deltas in
  SaturnScreenFabric.mouseDragged.
- Status effect icons keep 1.21.8's GUI atlas sprite; 1.21.9+ draws the
  texture instead (getIconId), as before.

Behaviour changes on 1.21.8: the cloak and hat read the Saturn player from
the render state (set by PlayerEntityRendererMixin, by UUID) instead of
looking it up by name, and the display-name null check applies.

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

README.MD

@@ -47,7 +47,7 @@ git clone --recurse-submodules https://git.selimaj.dev/saturnclientmc/saturnclie
 cd saturnclient
 
 ./gradlew buildAll                    # every version → build/allJars/
-./gradlew :1.21.11:build              # a single version (versions not on Stonecutter yet: :mc-1.21.8)
+./gradlew :1.21.11:build              # a single version (versions not on Stonecutter yet: :mc-1.21.7)
 ./gradlew :1.21.11:runClient          # launch a development client
 ```