v0.4.1 — schema emit consistency

Fixed

• durable.* audit categories now source swarm_class and topology from the durable run row instead of optional RunContext::metadata mirrors (#28). Previously these fields were typed string|null and would emit null whenever metadata was absent; they are now guaranteed non-null on all six durable.* categories (durable.completed, durable.failed, durable.cancelled, durable.paused, durable.checkpointed, durable.checkpointed_hierarchical). The change is implemented via two new helpers on DurableRunContext (swarmClassFor(), topologyFor()) that delegate to the existing requireRun() row lookup. Sinks that previously branched on null no longer need that branch.
• command.relay audit events with status: "error" now include exception_class (#32). The frozen schema in docs/audit-evidence-contract.md has documented this field since v0.4.0 but the emit in SwarmRelayCommand was missing it — the doc and the code now match.
• durable.cancelled audit events now include duration_ms (#33), bringing the cancelled category into parity with durable.completed and durable.failed. Computed via the existing DurableRunContext::durationMillisecondsFor() helper.
• webhook.signal_received audit events now include swarm_class (#29), bringing the category into parity with the sibling webhook.start_* categories. The field is plumbed through the new optional swarmClass property on DurableSignalResult — the property is nullable for backward compatibility with the public FakeDurableSwarmManager test fake, but is always populated in the production webhook path.

Changed

• docs/audit-evidence-contract.md frozen schema updated: dropped string|null annotations on durable.* swarm_class and topology, added duration_ms to durable.cancelled, added swarm_class to the webhook.signal_received envelope. All marked with (since v0.4.1) so sinks know when the new shape became reliable.