test(daemon): fence later work on open trace roots, not sequencer placeholders

A still-running mutating git command must be represented only by its
open trace root: it fences later entries of its family until it
finishes, and a root the reader has already seen finishing must keep
that fence, scoped to its own family, until the ingest worker processes
its final frame, including across socket EOF and when that frame yields
no command. Child git processes must not reclassify their root, and
placeholder entries in the sequencer are no longer expected.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>