fix: check rotate_segment return value; use uint32_t for CRC fields

- In persist_wal_async(), check rotate_segment() return value and propagate -EIO on failure (MEDIUM)
- Change last_crc and last_replicated_crc fields from uint64_t to uint32_t to match CRC32 width (LOW)

Both fixes address code review issues in commit b0a5fbb.

src/include/persist.h

@@ -89,8 +89,8 @@ struct persist_s {
     /* WAL state */
     int             wal_fd;
     uint64_t        current_lsn;
-    uint64_t        last_crc;
-    uint64_t        last_replicated_crc;
+    uint32_t        last_crc;
+    uint32_t        last_replicated_crc;
     uint64_t        term;
     uint32_t        seg_offset;
     uint64_t        checkpoint_lsn;