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]>

settings.gradle.kts

@@ -6,8 +6,7 @@ pluginManagement {
         mavenCentral()
     }
 
-    // Loom's version, for the Stonecutter versions' build.gradle.kts (the old mc-<v> projects set
-    // their own in versions/<v>/gradle.properties).
+    // Loom's version for build.gradle.kts, which can't read properties in its plugins block.
     val loom_version: String by settings
     plugins {
         id("net.fabricmc.fabric-loom-remap") version loom_version
@@ -24,7 +23,7 @@ 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("1.21.5", "1.21.6", "1.21.7", "1.21.8", "1.21.9", "1.21.10", "1.21.11")
+val stonecutterVersions = listOf("1.21.4", "1.21.5", "1.21.6", "1.21.7", "1.21.8", "1.21.9", "1.21.10", "1.21.11")
 
 stonecutter {
     create(rootProject) {
@@ -32,12 +31,3 @@ stonecutter {
         vcsVersion = "1.21.11"
     }
 }
-
-// Versions not moved to Stonecutter yet still build from their own copy in versions/<v>/src,
-// as projects named mc-<v>.
-file("versions").listFiles { file -> file.isDirectory && file.name !in stonecutterVersions }!!
-    .sortedBy { it.name }
-    .forEach { dir ->
-        include("mc-${dir.name}")
-        project(":mc-${dir.name}").projectDir = dir
-    }