Optimize launcher preparation with precompiled, batched exporters

Compile and validate original exporter bundles during release builds, cache development compilation, and reuse initialized JVMs and collision data during preparation. Keep legacy preparers compatible while current releases need only a Java runtime.

Co-Authored-By: Codex <noreply@openai.com>

.github/workflows/release.yml

@@ -77,9 +77,39 @@ jobs:
           # A dry run builds whatever the decision, to test the builds.
           if [[ "${{ steps.mode.outputs.dry_run }}" == true ]]; then echo "release=true" >> "$GITHUB_OUTPUT"; fi
 
-  build:
+  exporters:
     needs: decide
     if: needs.decide.outputs.release == 'true'
+    # This runner holds the maintainer's own pinned Minecraft jars. Only our classes leave it.
+    runs-on: ${{ vars.MAC_RUNNER || 'ludovics-mac-mini' }}
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ needs.decide.outputs.head }}
+          persist-credentials: false
+          clean: true
+      - uses: dtolnay/rust-toolchain@stable
+      - run: bash tools/ci/runner-env.sh
+      - run: bash tools/ci/vanilla-data.sh
+      - name: Compile and validate original exporters against the pinned jars
+        shell: bash
+        run: |
+          set -euo pipefail
+          bundle=$(target/release/horizon-prepare compile-exporters --root "$PWD" | tail -1)
+          mkdir -p exporters
+          cp -R "$bundle/server" "$bundle/client" "$bundle/manifest.json" exporters/
+          python3 tools/check_prepare_exporters.py --binary target/release/horizon-prepare
+      - uses: actions/upload-artifact@v4
+        with:
+          name: original-exporters
+          path: exporters/
+          if-no-files-found: error
+          retention-days: 3
+
+  build:
+    needs: [decide, exporters]
+    if: needs.decide.outputs.release == 'true'
     name: build ${{ matrix.name }}
     strategy:
       fail-fast: false
@@ -218,6 +248,10 @@ jobs:
           path: ${{ runner.temp }}/smoke-test.json
           if-no-files-found: error
           retention-days: 3
+      - uses: actions/download-artifact@v4
+        with:
+          name: original-exporters
+          path: exporters
       - name: Package
         run: |
           set -euo pipefail