chore(deps): update all libraries and Gradle to latest stable versions

Bump the version catalog, Gradle wrapper, and convention plugins to the
latest stable releases. Verified with `./gradlew assembleDebug test`
(BUILD SUCCESSFUL, 21 unit tests pass).

Toolchain:
- Gradle 9.1.0 -> 9.5.1
- AGP 9.0.1 -> 9.2.1
- Kotlin 2.3.20 -> 2.4.0

Libraries:
- androidx-core 1.18.0 -> 1.19.0, appcompat 1.7.0 -> 1.7.1,
  fragment 1.8.5 -> 1.8.9, navigation 2.9.0 -> 2.9.8
- compose-bom 2026.03.01 -> 2026.05.01, material 1.12.0 -> 1.14.0
- coroutines 1.10.2 -> 1.11.0, serialization 1.8.1 -> 1.11.0,
  collections-immutable 0.3.8 -> 0.5.0
- koin 4.1.0 -> 4.2.1, ktor 3.1.3 -> 3.5.0, coil 3.1.0 -> 3.5.0
- JUnit 5.11.4 -> 6.1.0, turbine 1.2.0 -> 1.2.1, mockk 1.14.3 -> 1.14.11

Required side-effect:
- compileSdk 36 -> 37 (mandated by androidx.core 1.19.0); targetSdk
  left at 36.

Also refresh stale JUnit 5 / AGP 9.0 / compileSdk 36 references in the
README and convention-plugin docs.

README.md

@@ -36,17 +36,17 @@ a small MVVM *About* screen for contrast, and a dedicated **error-handling demo*
 | Concern | Choice |
 |---|---|
 | Build | Multi-module Gradle + `:build-logic` **convention plugins**; a single **version catalog** (`gradle/libs.versions.toml`) is the only place versions live |
-| Toolchain | AGP 9.0.1, Kotlin 2.3.20, Gradle 9.1, `compileSdk`/`targetSdk` 36, `minSdk` 24, Java 17 |
+| Toolchain | AGP 9.2.1, Kotlin 2.4.0, Gradle 9.5.1, `compileSdk` 37 / `targetSdk` 36, `minSdk` 24, Java 17 |
 | UI | Jetpack Compose (Material 3) + one classic **Views/XML** renderer |
-| DI | Koin 4.1 (constructor DSL) |
+| DI | Koin 4.2 (constructor DSL) |
 | Networking | Ktor (OkHttp engine) + KotlinX Serialization |
 | Images | Coil 3 |
 | Navigation | type-safe Compose Navigation (`@Serializable` routes) |
 | Logging | Timber |
 | Async | Coroutines + Flow |
-| Testing | JUnit 5, MockK, Turbine, AssertK, `kotlinx-coroutines-test`, Ktor `MockEngine`, Compose UI test |
+| Testing | JUnit 6, MockK, Turbine, AssertK, `kotlinx-coroutines-test`, Ktor `MockEngine`, Compose UI test |
 
-> **AGP 9 gotcha:** AGP 9.0 has **built-in Kotlin**. Applying `com.android.application`/`library`
+> **AGP 9 gotcha:** AGP 9.2 has **built-in Kotlin**. Applying `com.android.application`/`library`
 > auto-applies the Kotlin Android plugin, so the convention plugins must **not** apply
 > `org.jetbrains.kotlin.android` themselves. Source lives in `src/main/kotlin`.
 
@@ -276,12 +276,12 @@ both resolve the **same** `CharacterListViewModel` class and supply its `SavedSt
 
 ## Testing
 
-Tests prove the architecture, not just the code. Stack: **JUnit 5**, **MockK**, **Turbine** (Flow),
+Tests prove the architecture, not just the code. Stack: **JUnit 6**, **MockK**, **Turbine** (Flow),
 **AssertK**, `kotlinx-coroutines-test`, Ktor **`MockEngine`**, and Compose UI test.
 
 | What | Where | Kind |
 |---|---|---|
-| `GetCharactersPageUseCase` | `:feature:characters:domain` `src/test` | pure JVM, JUnit 5 |
+| `GetCharactersPageUseCase` | `:feature:characters:domain` `src/test` | pure JVM, JUnit 6 |
 | `CharacterListViewModel`, `CharacterDetailViewModel` | `:feature:characters:presentation` `src/test` | JVM unit, MockK + Turbine + `SavedStateHandle` |
 | `NetworkCharacterRepository` | `:feature:characters:data` `src/test` | JVM unit, Ktor `MockEngine` |
 | `CharacterListScreen` (robot) | `:feature:characters:presentation-compose` `src/androidTest` | instrumented Compose UI |
@@ -301,7 +301,7 @@ Conventions demonstrated:
   reads as a scenario; it asserts a rendered item, the empty/error states, and that a tap fires the
   right `Action`.
 
-> **JUnit 5 on AGP 9:** the `de.mannodermaus.android-junit5` Gradle plugin targets AGP 8.x, so this
+> **JUnit 6 on AGP 9:** the `de.mannodermaus.android-junit5` Gradle plugin targets AGP 8.x, so this
 > repo doesn't use it. `AndroidUnitTest` extends Gradle's `Test`, so the `architecture.android.unit.test`
 > convention plugin just calls `useJUnitPlatform()` and adds the `unit-test` bundle - including the
 > `junit-platform-launcher`, which Gradle 9 no longer bundles.
@@ -323,7 +323,7 @@ possible but intentionally omitted (the VM logic is already covered by the share
 # Build
 ./gradlew assembleDebug                 # build the debug APK
 ./gradlew projects                      # print the module tree
-./gradlew test                          # all JVM unit tests (JUnit 5)
+./gradlew test                          # all JVM unit tests (JUnit 6)
 ./gradlew :feature:characters:presentation-compose:connectedDebugAndroidTest   # Compose UI test (needs a device)
 ```
 
@@ -339,7 +339,7 @@ android docs search "<topic>"           # search authoritative Android docs
 ```
 
 Requires JDK 17+ (the Gradle build pins a Java 17 toolchain) and the Android SDK
-(`compileSdk 36`, `minSdk 24`).
+(`compileSdk 37`, `minSdk 24`).
 
 ---