Combine measurement and stddev columns

[jhawthorn]

In #479 (cc @eightbitraptor @k0kubun) we were discussing the horizontal space. We can tighten the formatting and (IMO) improve readability by combining the measurement and stddev% into a single column. I also like this because the previous column was just named stddev (%) without specifying which measurement it was associated with.

ruby-base: ruby 4.1.0dev (2026-02-17T02:38:25Z master 125b3ad93f) +PRISM [x86_64-linux]
ruby-test: ruby 4.1.0dev (2026-02-17T04:59:51Z gsub_perf b7fbb4a816) +PRISM [x86_64-linux]

--------------  --------------  --------------  -----------------  -------------------
bench           ruby-base (ms)  ruby-test (ms)  ruby-test 1st itr  ruby-base/ruby-test
rubocop                    N/A             N/A                N/A                  N/A
activerecord      161.1 ± 1.3%    159.0 ± 0.8%              1.005                1.014
chunky-png        451.5 ± 0.3%    442.9 ± 0.4%              1.021                1.020
erubi-rails       743.4 ± 0.6%    748.0 ± 0.6%              1.019                0.994
hexapdf          1450.9 ± 1.0%   1480.9 ± 0.5%              0.992                0.980
liquid-c           35.8 ± 5.6%     35.1 ± 4.8%              1.015                1.021
liquid-compile     31.8 ± 1.8%     32.5 ± 7.4%              1.048                0.977
liquid-render      90.3 ± 0.6%     92.0 ± 0.5%              0.999                0.981
lobsters          646.8 ± 0.6%    647.1 ± 0.5%              1.013                1.000
mail               76.9 ± 1.2%     75.8 ± 1.0%              1.011                1.014
psych-load       1228.7 ± 1.5%   1186.5 ± 0.4%              1.023                1.036
railsbench       1327.3 ± 1.5%   1296.9 ± 1.0%              1.015                1.024
ruby-lsp           98.5 ± 0.8%     98.1 ± 0.6%              1.097                1.004
sequel             28.3 ± 1.2%     28.2 ± 0.6%              0.999                1.003
shipit           1162.2 ± 2.4%   1147.5 ± 2.2%              1.002                1.013
--------------  --------------  --------------  -----------------  -------------------

If folks are generally in favour of this I'll rebase it over #479 and make sure that looks good.

[k0kubun]

If folks are generally in favour of this

I am 👍

[eightbitraptor]

If folks are generally in favour of this

I am too, this looks great.