Show --zjit-stats in the summary table

[k0kubun]

When --zjit-stats=stat1,stat2 is passed, add those stats as columns in the summary table in addition to printing per-iteration diffs. This generalizes the approach so any ZJIT stat can be surfaced in the summary without needing a dedicated flag per stat.

Example

$ ./run_benchmarks.rb lobsters --chruby 'before --zjit;after --zjit' --zjit-stats=code_region_bytes,zjit_alloc_bytes --no-sudo --warmup=1 --bench=1
...

before: ruby 4.1.0dev (2026-03-26T07:27:31Z master c5ab2114df) +ZJIT +PRISM [x86_64-linux]
after: ruby 4.1.0dev (2026-03-26T07:27:46Z zjit-lightweight-f.. a32ce82bf2) +ZJIT +PRISM [x86_64-linux]

--------  ------------  -----------------  ----------------  ------------  -----------------  ----------------  -------------  ------------
bench      before (ms)  code_region_bytes  zjit_alloc_bytes    after (ms)  code_region_bytes  zjit_alloc_bytes  after 1st itr  before/after
lobsters  959.9 ± 0.0%          9,244,672        14,301,972  956.8 ± 0.0%          9,990,144        15,289,608          0.988         1.003
--------  ------------  -----------------  ----------------  ------------  -----------------  ----------------  -------------  ------------