Full-history capacity checks occasionally hit the 20-second CI timeout while reopening a document. Use smaller valid filler events on a separate unlinked thread, preserving the 4,096-event boundary, real persistence/restart, linked card state, and every existing assertion.

Local restart cases fell from roughly 7.1 seconds to 4.9–5.3 seconds. Production behavior and test timeouts are unchanged.

Validation: all five capacity tests and 57 assertions pass; types and CI checks pass; independent review approved the fixture.