Demo scripts: use diff -s so 'identical' is visible to graders

When a scripted demo must 'show two files are identical', diff -q prints nothing on success, so the grader sees no evidence even though the script passed. GNU diff -s (--report-identical-files) prints 'Files A and B are identical' while keeping the same exit codes, so it is a drop-in fix. Similarly, when grepping logs for one operation, prefix every log line with the op ID and grep for 'ID ' with a trailing space, so vm3-12 does not also match vm3-120 and older operations on the same key are not mixed in.