Fix MariaDB query cache efficiency underreporting

[Copilot]

On MariaDB, Com_select already includes Qcache_hits (verified: Qcache_hits + Qcache_inserts + Qcache_not_cached ≈ Com_select). The old MySQL formula Qcache_hits / (Com_select + Qcache_hits) double-counts hits in the denominator, producing ~45% where the real hit ratio is ~84%.

Changes

• calculations(): Use Qcache_hits / Com_select for MariaDB; keep Qcache_hits / (Com_select + Qcache_hits) for MySQL. Uses the existing $is_mariadb local variable (computed before the function strips the -MariaDB suffix from $myvar{'version'}). Stores result in $mycalc{'is_mariadb'} for downstream use.

• mysql_stats(): Use $mycalc{'is_mariadb'} to display the correct denominator (Com_select vs Com_select + Qcache_hits).

• tests/issue_927.t: New test asserting the MariaDB formula yields ~84.4% and the MySQL formula is unchanged for non-MariaDB versions.

# MariaDB (Com_select already includes hits)
$mycalc{'query_cache_efficiency'} = sprintf("%.1f",
    ($mystat{'Qcache_hits'} / $mystat{'Com_select'}) * 100
);

# MySQL (hits not counted in Com_select)
$mycalc{'query_cache_efficiency'} = sprintf("%.1f",
    ($mystat{'Qcache_hits'} / ($mystat{'Com_select'} + $mystat{'Qcache_hits'})) * 100
);

Note: The version-stripping gotcha — calculations() runs $myvar{'version'} =~ s/(.+)-.*?$/$1/ before reaching the query cache block, so $myvar{'version'} =~ /mariadb/i would silently fail there. The fix uses $is_mariadb set prior to the strip.