chore(android): audit-response wave — JUnit launcher, compileSdk 36, CC warn, image :36, Gradle cache

Fixes uncovered by the 2026-05-22 build-config audit + the Gradle 9 +
JUnit Platform launcher requirement that just surfaced in CI:

  - testRuntimeOnly junit-platform-launcher: Gradle 9 no longer auto-
    injects it; tests fail with "Failed to load JUnit Platform" without
    an explicit dep.
  - compileSdk + targetSdk 35 -> 36: AGP 9 defaults to 36 and warns on
    lower values; CI was also auto-downloading build-tools 36 at
    runtime (now pre-installed in ci-android:36).
  - container.image bumped to ci-android:36 to match.
  - configuration-cache.problems=warn in gradle.properties: detekt 2.0-
    alpha + ktlint Gradle plugin have CC compat holes; warn rather
    than fail.
  - androidTest dep parity: kotlin("test") + kotlinx-coroutines-test
    added (was on testImplementation only).
  - CI: actions/cache@v4 for ~/.gradle/{caches,wrapper} + ~/.kotlin,
    keyed on gradle-wrapper.properties + libs.versions.toml + the
    *.gradle.kts files. Saves ~3 min per run after warm-up.

Deferred (with trigger conditions, will land when needed): Hilt
testing artifacts + HiltTestRunner (first @HiltAndroidTest), Room
Gradle plugin + schemaDirectory (Phase 4.2), Coil 3 ImageLoader
factory sharing OkHttp (Phase 5.4), MainDispatcherRule test utility
(Phase 5.3), JUnit-5/4 split for instrumented (Phase 5+),
NetworkSecurityConfig (pre-cutover Phase 14).

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

.forgejo/workflows/android.yml

@@ -25,7 +25,7 @@ jobs:
     # android-ci once the operator registers that runner label.
     runs-on: flutter-ci
     container:
-      image: git.fabledsword.com/bvandeusen/ci-android:35
+      image: git.fabledsword.com/bvandeusen/ci-android:36
 
     defaults:
       run:
@@ -35,6 +35,19 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Cache Gradle dirs
+        # Resolved deps + Gradle distribution + Kotlin daemon caches.
+        # Saves ~3 min per CI run after the first warm-up.
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+            ~/.kotlin
+          key: gradle-${{ runner.os }}-${{ hashFiles('android/gradle/wrapper/gradle-wrapper.properties', 'android/gradle/libs.versions.toml', 'android/**/*.gradle.kts') }}
+          restore-keys: |
+            gradle-${{ runner.os }}-
+
       - name: Make gradlew executable
         run: chmod +x ./gradlew
 
@@ -67,7 +80,7 @@ jobs:
     if: startsWith(github.ref, 'refs/tags/v')
     runs-on: flutter-ci
     container:
-      image: git.fabledsword.com/bvandeusen/ci-android:35
+      image: git.fabledsword.com/bvandeusen/ci-android:36
 
     defaults:
       run: