[backend] Clarification round-trip lifecycle test

[KayleeWilliams]

Why

Clarification pauses the clock per EU policy rules (clarificationEffect: "stop_clock"). If this silently drifts, deadlines are wrong - a legally significant bug. No integration test exercises the full round-trip today.

Scope

• E2E test: capture -> verification -> in_progress -> clarification_request -> clarification_receive -> fulfil
• Assert clock segments reflect the pause window with countsTowardDeadline: false
• Assert deadline shifts by exactly the pause duration
• Assert timeline events recorded correctly
• Run with EU (stops clock) and UK (no-stop) packs to verify policy-driven divergence

Priority: P1 - beta blocker