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>

android/app/build.gradle.kts

@@ -14,12 +14,12 @@ plugins {
 
 android {
     namespace = "com.fabledsword.minstrel"
-    compileSdk = 35
+    compileSdk = 36
 
     defaultConfig {
         applicationId = "com.fabledsword.minstrel"
         minSdk = 26
-        targetSdk = 35
+        targetSdk = 36
         versionCode = 1
         versionName = "0.1.0-native"
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
@@ -160,9 +160,17 @@ dependencies {
     // kotlin.test for assertEquals/assertNull/etc. — version managed by
     // the applied Kotlin plugin so no explicit version pin needed.
     testImplementation(kotlin("test"))
+    // Gradle 9 no longer auto-injects the JUnit Platform launcher; must
+    // be declared explicitly on the runtime classpath for useJUnitPlatform()
+    // to discover tests.
+    testRuntimeOnly(libs.junit.platform.launcher)
 
     androidTestImplementation(platform(libs.compose.bom))
     androidTestImplementation(libs.compose.ui.test)
+    // androidTest dep parity with the unit-test side; needed once we have
+    // instrumented tests that consume the same APIs.
+    androidTestImplementation(kotlin("test"))
+    androidTestImplementation(libs.kotlinx.coroutines.test)
     debugImplementation(libs.compose.ui.test.manifest)
 }