Fix binary units usage in checkpoint statistics

- Change "Checkpoint MB/sec" to "Checkpoint MiB/sec"
- Complies with project binary units standards (base-2)
- Calculation uses 1024.0 * 1024, so MiB is accurate
- Applies to both PostgreSQL 17+ and <17 versions

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: NikolayS

sql/0_node.sql

@@ -58,7 +58,7 @@ select 'Forced Checkpoints', (
   from pg_stat_checkpointer
 )
 union all
-select 'Checkpoint MB/sec', (
+select 'Checkpoint MiB/sec', (
   select round((nullif(buffers_written::numeric, 0) /
     ((1024.0 * 1024 /
       (current_setting('block_size')::numeric))
@@ -75,7 +75,7 @@ select 'Forced Checkpoints', (
   from pg_stat_bgwriter
 )
 union all
-select 'Checkpoint MB/sec', (
+select 'Checkpoint MiB/sec', (
   select round((nullif(buffers_checkpoint::numeric, 0) /
     ((1024.0 * 1024 /
       (current_setting('block_size')::numeric))