fix: no wait after kill before compression and pidstat metric normalization

[k-rister]

Bugs Found

1. Race condition on stop (MEDIUM)

sysstat-stop lines 9-15: Killed processes may still be flushing data to disk when xz compression begins immediately after — race condition can corrupt or truncate the last sample.

2. Inconsistent pidstat metric normalization (MEDIUM)

sysstat-post-process.py line 509: pidstat CPU values are in the range 0-100 while mpstat normalizes to 0-1 for the same metric type names. Inconsistent metric units within the same tool.

3. Typo in filename (LOW)

sysstat-stop line 17: sysstst-pids.txt should be sysstat-pids.txt.