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]>

settings.gradle.kts

@@ -5,16 +5,15 @@ pluginManagement {
         gradlePluginPortal()
         mavenCentral()
     }
-
-    // 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
-    }
 }
 
 plugins {
+    // Downloads the Java toolchain a version needs (Java 25 for 26.1+) when it isn't installed.
+    id("org.gradle.toolchains.foojay-resolver-convention") version "1.0.0"
     id("dev.kikugie.stonecutter") version "0.9.8"
+    // Picks the Loom variant per version: remapping for 1.21.x, plain for the unobfuscated 26.1+.
+    // Its version is loomx.loom_version in gradle.properties.
+    id("dev.kikugie.loom-back-compat") version "0.4.2"
 }
 
 rootProject.name = "saturn-client"
@@ -23,7 +22,9 @@ include("common")
 
 // Versions built by Stonecutter from the shared src/ (see stonecutter.gradle.kts). Each one's
 // versions/<v>/gradle.properties holds its Minecraft, Fabric Loader and Fabric API versions.
-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")
+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) {