Diffs member WALs by (lsn -> term, pcrc) over the committed prefix, exits non-zero
on any committed-prefix divergence with the diverging LSN + per-member entry
identities. Reuses the canonical waldump decoder via a new per-entry visitor seam
(no second WAL parser). Detects same-LSN/diff-term and same-LSN/diff-content
divergences (the bug-851 class) that an LSN-range check would mask.
