fix(compress): tighten wording and test timing after second review pass

The ENOLCK-exclusion comment claimed the errno "means" transient kernel
lock-record exhaustion; it can also signal a permanent NFS remote-locking
failure, so the comment now says it isn't a reliable capability signal
rather than asserting a single cause.

Patch LOCK_WAIT_SECONDS/LOCK_POLL_INTERVAL down in the two new file_lock
error-path tests, matching every other lock test in this file, so a
misclassification regression fails in seconds instead of spinning out the
real 900s budget.

Moved the "checks run before the lock" comment one line down to sit above
the checks themselves rather than above the MAX_FILE_SIZE assignment, and
added the missing basic-checks step to the README pipeline diagram, which
otherwise showed lock acquisition as the very first step.