The DEFAULT ReactiveDag.CoordinationWriter — spine-only, over the configured
tuple table via ReactiveDag.Tuple. Suitable for a host with no extension
columns on its coordination tuple. A host with extensions (cascade's
source_ref/tombstoned_at, the portal's strength) configures its own
writer that does the spine + extension write in one atomic upsert.
put/3 returns the boolean CHANGED signal (ReactiveDag.Tuple.put_changed/3):
true for a new row or a status flip, false for a re-put of the same
status — so ops using the default writer propagate only real changes.
tombstone/2 here has no retain policy, so it falls back to delete/2 (a
spine with no tombstoned_at column can't retain).