fix(waldump): decode WAL_CONFIG_MARKER, distinguish trimmed gaps from real loss

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.

src/controller/testsuite/waldump.h

@@ -29,6 +29,13 @@ typedef struct {
     uint32_t n_dup_identical; /* of n_dup_submits, those whose payload is
                                * byte-identical to the prior entry */
     int      crc_broken;    /* nonzero if a CRC/chain break truncated a segment */
+    int      leading_gap_explained; /* nonzero if the leading (prefix) gap's
+                                     * upper LSN is covered by a sibling
+                                     * data/snap/snap-<hex>.capnp file — i.e.
+                                     * normal checkpoint-triggered WAL trim
+                                     * (persist.c trim_wal_segments), not
+                                     * genuine loss. 0 if there was no leading
+                                     * gap, or no covering snapshot was found. */
 } waldump_stats_t;
 
 /* One physically-present, CRC-valid WAL entry, surfaced to a visitor so a