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>

src/main/resources/fabric.mod.json

@@ -16,9 +16,9 @@
     "client": [ "com.ejclaw.videoplayer.VideoPlayerClient" ]
   },
   "depends": {
-    "fabricloader": ">=0.16.0",
+    "fabricloader": ">=0.19.0",
     "fabric-api": "*",
     "minecraft": "${target_minecraft}",
-    "java": ">=21"
+    "java": ">=25"
   }
 }