ci(audience-sdk): add PlayMode test workflow [SDK-148]

Four-cell matrix builds the audience sample app and runs PlayMode tests
inside the built player across both backends:

  - StandaloneWindows64 / IL2CPP / windows-latest / Unity 2021.3.45f2
  - StandaloneWindows64 / Mono2x / windows-latest / Unity 2021.3.45f2
  - StandaloneOSX     / IL2CPP / macos-latest   / Unity 2021.3.45f2
  - StandaloneOSX     / Mono2x / macos-latest   / Unity 2021.3.45f2

Backend selection per cell goes through AUDIENCE_SCRIPTING_BACKEND env
var consumed by the ScriptingBackendOverride Editor script.

Notes:
  - Both targets use Windows / macOS hosts because game-ci can cross-
    build Windows IL2CPP players on Linux but the runner can't launch
    them to drive PlayMode tests; the host OS must match the player
    target.
  - Unity 2021.3.45f2 (not 2021.3.26f1 as originally planned) avoids a
    zlib-vs-macOS-15-SDK toolchain incompatibility that breaks the
    IL2CPP build under newer Xcode CLT (Unity zutil.h:147 macros fdopen
    to NULL, collides with stdio.h fdopen function declaration).
  - Tests live-fire to sandbox via AUDIENCE_TEST_PUBLISHABLE_KEY repo
    secret. Skipped on fork PRs to avoid secret leakage.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

.github/workflows/test-audience-il2cpp.yml

@@ -0,0 +1,74 @@
+name: Audience SDK — PlayMode (IL2CPP + Mono)
+
+on:
+  pull_request:
+    paths:
+      - 'src/Packages/Audience/**'
+      - 'examples/audience/**'
+      - '.github/workflows/test-audience-il2cpp.yml'
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  playmode:
+    if: github.event.pull_request.head.repo.fork == false
+    name: ${{ matrix.target }} / ${{ matrix.backend }} / Unity ${{ matrix.unity }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          # Windows players must run on a Windows host — game-ci can
+          # cross-build the player on Linux but PlayMode tests need to launch
+          # the player, which requires the host OS to match the target.
+          - target: StandaloneWindows64
+            backend: IL2CPP
+            unity: 2021.3.45f2
+            os: windows-latest
+          - target: StandaloneWindows64
+            backend: Mono2x
+            unity: 2021.3.45f2
+            os: windows-latest
+          - target: StandaloneOSX
+            backend: IL2CPP
+            unity: 2021.3.45f2
+            os: macos-latest
+          - target: StandaloneOSX
+            backend: Mono2x
+            unity: 2021.3.45f2
+            os: macos-latest
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          lfs: true
+      - run: git lfs pull
+      - uses: actions/cache@v3
+        with:
+          path: examples/audience/Library
+          key: Library-${{ matrix.backend }}-${{ matrix.target }}-${{ matrix.unity }}-${{ hashFiles('examples/audience/Assets/**', 'examples/audience/Packages/**', 'examples/audience/ProjectSettings/**', 'src/Packages/Audience/**') }}
+          restore-keys: |
+            Library-${{ matrix.backend }}-${{ matrix.target }}-${{ matrix.unity }}
+            Library-${{ matrix.backend }}-${{ matrix.target }}
+      - uses: game-ci/unity-test-runner@v4
+        id: tests
+        env:
+          UNITY_EMAIL: ${{ secrets.UNITY_EMAIL }}
+          UNITY_PASSWORD: ${{ secrets.UNITY_PASSWORD }}
+          UNITY_SERIAL: ${{ secrets.UNITY_SERIAL }}
+          AUDIENCE_TEST_PUBLISHABLE_KEY: ${{ secrets.AUDIENCE_TEST_PUBLISHABLE_KEY }}
+          AUDIENCE_SCRIPTING_BACKEND: ${{ matrix.backend }}
+        with:
+          unityVersion: ${{ matrix.unity }}
+          projectPath: 'examples/audience'
+          testMode: 'PlayMode'
+          targetPlatform: ${{ matrix.target }}
+          githubToken: ${{ secrets.GITHUB_TOKEN }}
+      - uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: playmode-${{ matrix.backend }}-${{ matrix.target }}-${{ matrix.unity }}
+          path: ${{ steps.tests.outputs.artifactsPath }}