flake: emit Unity bundle zip; add Gitea Actions CI/release; publish VPM index

- flake.nix: install the libaac cdylib alongside aac-dump and zip it with
  the C# Unity packages as the default package
- .gitea/workflows: build+package on push/PR, publish release on v* tags
- vpm: commit the repository index and packaging script, with VPM_INDEX and
  VPM_INDEX_URL overrides for generating a local listing

.gitea/workflows/ci.yml

@@ -0,0 +1,41 @@
+name: ci
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/install-nix-action@v27
+        with:
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+
+      # Builds the cdylib and runs the cargo tests (doCheck).
+      - name: Build Rust core
+        run: nix build .#aac --print-build-logs
+
+      # Rehearses the VPM release: stages the C# package, bundles libaac, updates vpm/index.json.
+      - name: Package VPM zip
+        run: nix develop -c ./vpm/package.sh
+
+      # The Unity bundle produced by the flake itself.
+      - name: Build Unity bundle
+        run: nix build .#default --print-build-logs
+
+      - name: Collect artifacts
+        run: |
+          mkdir -p dist
+          cp -L result dist/animator-as-crab-bundle.zip
+          cp vpm/dist/*.zip dist/
+
+      - uses: actions/upload-artifact@v3
+        with:
+          name: animator-as-crab
+          path: dist/*.zip