wal_type_name() never had a case for WAL_CONFIG_MARKER (type 50) -- every
config-change marker entry printed as UNKNOWN instead of a real, defined
type. Added the case plus payload decode (tag/sha256) matching config_repl.c's
config_marker_encode/_decode byte layout, hand-decoded here rather than
linking config_repl.c into xce_test's link graph.
Gap detection also had no way to tell "committed prefix missing!" (real
loss) apart from a normal checkpoint-triggered WAL trim (persist.c's
trim_wal_segments, floor-guarded so it never removes a segment a standby
still needs) -- every long-running node's WAL, once checkpointed, would
false-alarm on every inspection. Cross-checks the sibling data/snap/
directory (snapshot LSN is literally in the filename, snap-<hex>.capnp)
and downgrades the message when a snapshot covers the gap.
Adds waldump_stats_t.leading_gap_explained and 3 new tests plus one
assertion on the existing gap test.
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.
