build: switch toolchain to MC 26.1.2 (intermediary retired)

- net.fabricmc.fabric-loom 1.16-SNAPSHOT (no remap; MC 26.1+ ships unobfuscated)
- gradle.properties: minecraft_version=26.1.2, loader=0.19.2, fabric-api=0.149.0+26.1.2
- Java 25 toolchain
- fabric.mod.json: fabricloader>=0.19.0, java>=25
- Drop multi-version build script + matrix CI (single-target now)
- Backup of 1.21.6/7/8 working tree preserved on mc-1.21.x branch

Source migration to Mojmap names is in progress on follow-up commits;
this commit alone will not build until source files are ported.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

build.gradle

@@ -1,5 +1,5 @@
 plugins {
-    id 'fabric-loom' version '1.16.2'
+    id 'net.fabricmc.fabric-loom' version "${loom_version}"
     id 'maven-publish'
     id 'java'
 }
@@ -16,11 +16,16 @@ repositories {
     maven { url = 'https://maven.fabricmc.net/' }
 }
 
+loom {
+    // Intentionally empty — MC 26.1+ ships unobfuscated, so the new loom does not remap.
+}
+
 dependencies {
+    // No mappings dep — Mojang ships official names since 26.1, intermediary is gone.
     minecraft "com.mojang:minecraft:${project.minecraft_version}"
-    mappings "net.fabricmc:yarn:${project.yarn_mappings}:v2"
-    modImplementation "net.fabricmc:fabric-loader:${project.loader_version}"
-    modImplementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
+
+    implementation "net.fabricmc:fabric-loader:${project.loader_version}"
+    implementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
 }
 
 processResources {
@@ -28,27 +33,24 @@ processResources {
     inputs.property "mod_id", project.mod_id
     inputs.property "minecraft_version", project.minecraft_version
 
-    // Pin the fabric.mod.json's minecraft constraint to the build's exact target — keeps each
-    // multi-version jar from claiming compatibility it doesn't have.
-    def mc = project.minecraft_version
-    def target = "~${mc}"
-
     filesMatching("fabric.mod.json") {
-        expand "version": project.version, "mod_id": project.mod_id, "target_minecraft": target
+        expand "version": project.version,
+               "mod_id": project.mod_id,
+               "target_minecraft": "~${project.minecraft_version}"
     }
 }
 
 tasks.withType(JavaCompile).configureEach {
-    it.options.release = 21
+    it.options.release = 25
 }
 
 java {
     withSourcesJar()
-    sourceCompatibility = JavaVersion.VERSION_21
-    targetCompatibility = JavaVersion.VERSION_21
+    sourceCompatibility = JavaVersion.VERSION_25
+    targetCompatibility = JavaVersion.VERSION_25
 
     toolchain {
-        languageVersion = JavaLanguageVersion.of(21)
+        languageVersion = JavaLanguageVersion.of(25)
     }
 }