Build 1.21.4 with Stonecutter; every version now shares src/

Step 6 of #8. versions/1.21.4/src is gone, so all eight versions (1.21.4 to
1.21.11) build from src/.

1.21.4's differences from 1.21.5 are behind version comments:
- The end gateway render layer has no render pipeline before 1.21.5
  (ShaderUtils: the pipeline field only exists from 1.21.5, and the layer
  is a four-way chain).
- Hat head rotation, the NativeImageBackedTexture constructor, armor slots
  (inventory.armor vs getEquippedStack), MatrixStackMixin's Quaternionf,
  and inside the pre-1.21.6 implementations: prevHeadYaw, GUI lighting,
  ModelTransformationMode, item depth, setShaderTexture and the blur
  uniform call. Those are one-line version comments, since the pre-1.21.6
  blocks are already inside /* */ on newer versions.

1.21.5's drawItem wrapped its body in a second isEmpty check after already
returning early for empty stacks; the redundant check is gone, so 1.21.4
and 1.21.5 share the body.

Freelook's zoom-out (FreelookMod.shouldZoomOut in GameRendererMixin) only
existed on 1.21.4. It doesn't depend on the Minecraft version, so it's
back on every version.

With no old copies left, the mc-<v> fallback in settings.gradle.kts and
scripts/port.sh are removed, and README and VERSION_GUIDE describe the
single layout.

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-1.21.5)
+./gradlew :1.21.11:build              # a single version
 ./gradlew :1.21.11:runClient          # launch a development client
 ```
 
@@ -65,7 +65,7 @@ We welcome contributions of all kinds:
 
 If you're an artist or designer interested in helping with cosmetics or visual assets, we'd love to collaborate.
 
-Most features belong in `common`, which every Minecraft version shares. For code that differs between versions, see the [version guide](VERSION_GUIDE.md): it covers how the versions are laid out, porting a change to all of them with `scripts/port.sh`, and adding a new Minecraft version. Every pull request is compiled for all versions automatically.
+Most features belong in `common`, which every Minecraft version shares. For code that differs between versions, see the [version guide](VERSION_GUIDE.md): it covers how the versions share one copy of the code with Stonecutter, writing version-specific code, and adding a new Minecraft version. Every pull request is compiled for all versions automatically.
 
 ## Credits