debug: add fail-fast=false and --stacktrace to diagnose CI failures

All three MC version builds fail at different times (~25-30s), suggesting a
common configuration-phase failure. With fail-fast disabled and stacktrace
enabled, we can see the actual error for each version simultaneously.

https://claude.ai/code/session_01PHDS24JTjD32guufn2q2sj

Author: claude

.github/workflows/build.yml

@@ -5,6 +5,7 @@ jobs:
   build:
     runs-on: ubuntu-24.04
     strategy:
+      fail-fast: false
       matrix:
         include:
           - mc: "1.21"
@@ -26,7 +27,7 @@ jobs:
       - name: make gradle wrapper executable
         run: chmod +x ./gradlew
       - name: build mc${{ matrix.mc }}
-        run: ./gradlew :${{ matrix.mc }}:build
+        run: ./gradlew :${{ matrix.mc }}:build --stacktrace
       - name: capture build artifacts
         uses: actions/upload-artifact@v4
         with: