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]>

.gitea/workflows/check.yml

@@ -19,15 +19,20 @@ jobs:
       # The JDK comes from the runner's tool cache. `cache: gradle` stores Gradle's
       # dependencies, Loom's Minecraft cache and the build cache in the runner's
       # actions cache.
-      - name: Set up Java 21
+      # 1.21.x builds with Java 21 and 26.x with Java 25. The last version listed is the default
+      # (Gradle runs on it), and the Gradle commands below point toolchain detection at both.
+      - name: Set up Java 25 and 21
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 21
+          java-version: |
+            25
+            21
           cache: gradle
 
       - name: Compile all versions
         run: |
           chmod +x gradlew
           # --continue reports every version that fails, not just the first.
-          ./gradlew compileJava --no-daemon --stacktrace --continue
+          ./gradlew compileJava --no-daemon --stacktrace --continue \
+            -Porg.gradle.java.installations.fromEnv=JAVA_HOME_21_X64,JAVA_HOME_25_X64

.gitea/workflows/release.yml

@@ -47,11 +47,15 @@ jobs:
       # The JDK comes from the runner's tool cache. `cache: gradle` stores Gradle's
       # dependencies, Loom's Minecraft cache and the build cache in the runner's
       # actions cache.
-      - name: Set up Java 21
+      # 1.21.x builds with Java 21 and 26.x with Java 25. The last version listed is the default
+      # (Gradle runs on it), and the Gradle commands below point toolchain detection at both.
+      - name: Set up Java 25 and 21
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 21
+          java-version: |
+            25
+            21
           cache: gradle
 
       - name: Build all versions
@@ -59,7 +63,8 @@ jobs:
           chmod +x gradlew
           # buildAll remaps every Minecraft version (two at a time, to stay within the
           # Gradle heap) into build/allJars.
-          ./gradlew buildAll --no-daemon --stacktrace
+          ./gradlew buildAll --no-daemon --stacktrace \
+            -Porg.gradle.java.installations.fromEnv=JAVA_HOME_21_X64,JAVA_HOME_25_X64
 
       - name: Upload jars to the release
         env: