artifact: core/router/src/main/kotlin/com/correx/core/router/RouterContextBuilder.kt
purpose: Implement RouterContextBuilder — assembles a budget-constrained ContextPack from L2 memory (stage summaries), workflow status events, and conversation history, respecting L0 immutability and L2 eviction ordering.
depends_on: task-006, task-007
non_goals: Implementing inference routing, facade orchestration, or event writing
done_when: RouterContextBuilder.build() produces a ContextPack with correct layer assignments, budget enforcement, and L0 entries preserved under all conditions
