An old V1 server on an empty pre-V2 source can create its first thread
after ensureNoPendingLegacyThreads passes but before copySourceSlice reads
the source. The unfiltered copy takes the row, migrations and prune drop
it, and the run exits 0. Red test: the race is injected at the destination
state-dir makeDirectory, the first FileSystem call after the guard.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>