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>

.gitea/workflows/build.yml

@@ -1,4 +1,4 @@
-name: build-matrix
+name: build
 
 on:
   push:
@@ -9,33 +9,15 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - mc: "1.21.6"
-            yarn: "1.21.6+build.1"
-            fabric: "0.120.1+1.21.6"
-          - mc: "1.21.7"
-            yarn: "1.21.7+build.8"
-            fabric: "0.129.0+1.21.7"
-          - mc: "1.21.8"
-            yarn: "1.21.8+build.1"
-            fabric: "0.136.1+1.21.8"
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-java@v4
         with:
           distribution: temurin
-          java-version: 21
-      - name: Build (MC ${{ matrix.mc }})
-        run: |
-          ./gradlew --no-daemon \
-            -Pminecraft_version=${{ matrix.mc }} \
-            -Pyarn_mappings=${{ matrix.yarn }} \
-            -Pfabric_version=${{ matrix.fabric }} \
-            build
+          java-version: 25
+      - name: Build (MC 26.1.2)
+        run: ./gradlew --no-daemon build
       - uses: actions/upload-artifact@v4
         with:
-          name: video_player-mc${{ matrix.mc }}
+          name: video_player-mc26.1.2
           path: build/libs/*.jar