artifact: testing/deterministic/src/test/kotlin/RouterContextBuilderTest.kt
purpose: Test RouterContextBuilder — verify budget enforcement, L0 immutability, L2 oldest-first eviction, and layer classification.
depends_on: task-012, task-019
non_goals: Testing facade orchestration, testing inference routing
done_when: All context builder tests pass with ./gradlew :testing:deterministic:test --tests RouterContextBuilderTest
