test(runtime): guard event ordering across migrated runtimes

[dshipper]

An older event writer can fail with UNIQUE constraint failed: feed_events.event_order after a database migration. This regression pins the allocator already present on main against that failure.

The fixture includes an existing zero order and a gap, then interleaves two store connections across feeds. It checks global append order despite clock skew, feed isolation, and retries after reconnecting that preserve the original approval and completion receipt events.

Validation: the test reproduces the reported error on the pre-fix writer and passes on both the repaired runtime and current main. pnpm check passes: type checking, lint, and 216 tests; the local Supabase integration test is skipped.