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>

gradle.properties

@@ -1,14 +1,16 @@
 org.gradle.jvmargs=-Xmx2G
 org.gradle.parallel=true
+# Config cache disabled — new loom + IntelliJ aren't fully compatible yet
+org.gradle.configuration-cache=false
 
 # Mod
 mod_id=video_player
-mod_version=0.2.0
+mod_version=0.3.0
 maven_group=com.ejclaw.videoplayer
 archives_base_name=video_player
 
-# Minecraft / Fabric (1.21.6)
-minecraft_version=1.21.6
-yarn_mappings=1.21.6+build.1
+# Minecraft / Fabric (26.1.2 — single target, intermediary/Yarn retired)
+minecraft_version=26.1.2
 loader_version=0.19.2
-fabric_version=0.120.1+1.21.6
+loom_version=1.16-SNAPSHOT
+fabric_version=0.149.0+26.1.2