Export the run's evidence bundle

Builds the run's evidence bundle from what it recorded and returns it: a ZIP holding manifest.json (the run, counts, and every file's size and SHA-256), events.json (the whole timeline in sequence order), expectations.json (each expectation with its verdict once the run is finished), faults.json, resources.json, definition.json, every message under messages/ and every request body under requests/, named by sequence and event id. Bounded at 10,000 events and 256 MiB of payloads; the manifest says when a bound bit or when retention had already taken some bytes. With Accept: application/json the manifest alone is returned. Read-only: the bundle is what the caller can already read event by event.