Build with loom-back-compat and a Java toolchain per version

Prepares the build for 26.x, which ships unobfuscated and needs Java 25
(#39):

- loom-back-compat applies the remapping Loom to 1.21.x and would apply
  the plain one to 26.x. loom_version becomes loomx.loom_version.
- Each version compiles with its Java toolchain (21 for 1.21.x, 25 for
  26.x), and the mixin configs take their compatibility level from it.
  The foojay resolver downloads a missing JDK; CI installs 21 and 25.
- buildAll collects remapJar's output, or jar's where there's nothing
  to remap.
- fabric.mod.json depends on fabric-api, Fabric API's mod id; 26.x's
  jar no longer provides the old "fabric" alias.

Nothing changes for the 1.21.x jars.

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

stonecutter.gradle.kts

@@ -69,17 +69,22 @@ subprojects {
 
 tasks.register("buildAll") {
     group = "build"
-    description = "Builds every Minecraft version's remapped jar and collects them in build/allJars"
+    description = "Builds every Minecraft version's mod jar and collects them in build/allJars"
 
-    val remapJars = subprojects.flatMap { it.tasks.matching { task -> task.name == "remapJar" } }
-    dependsOn(remapJars)
+    // The mod jar is remapJar's output for 1.21.x, and jar's for 26.1+, which isn't obfuscated.
+    val modJars = subprojects.filter { it.name != "common" }.flatMap { project ->
+        val unobfuscated = project.name.substringBefore('.').toInt() >= 26
+        val task = if (unobfuscated) "jar" else "remapJar"
+        project.tasks.matching { it.name == task }
+    }
+    dependsOn(modJars)
 
     doLast {
         val outputDir = layout.buildDirectory.dir("allJars").get().asFile
         outputDir.mkdirs()
-        remapJars.map { (it as AbstractArchiveTask).archiveFile.get().asFile }
+        modJars.map { (it as AbstractArchiveTask).archiveFile.get().asFile }
             .filter { it.exists() && !it.name.contains("sources") }
             .forEach { it.copyTo(outputDir.resolve(it.name), overwrite = true) }
-        println("All remapped jars collected in: $outputDir")
+        println("All mod jars collected in: $outputDir")
     }
 }