Add job size benchmark script + on-demand CI workflow

[TomasVotruba]

Adds a benchmark that measures Rector wall time across parallel job sizes, plus a manual CI workflow to run it.

Script

scripts/rector-jobsize-benchmark.php runs cold Rector dry-runs across a list of job sizes and prints a markdown table of wall time, slowdown vs fastest, exit code, and file counts. It is a standalone dev tool and is excluded from PHPStan.

CI workflow

.github/workflows/jobsize_benchmark.yaml runs on workflow_dispatch only (no per-PR cost). Sizes default to 50-200 by 25 and are overridable from the run dialog. Results go to the job summary and an uploaded artifact.

Measurements

Both runs analysed the same 2620 files, every run cold, one sample per size.

Remote (GitHub runner, 4 cores)

jobSize	wall	vs fastest
16	48.8 s	1.40x
50	39.3 s	1.13x
100	34.9 s	1.00x
150	35.6 s	1.02x
300	36.2 s	1.04x

Local (24 cores)

jobSize	wall	vs fastest
50	49.1 s	1.16x
100	44.7 s	1.06x
130	42.2 s	1.00x
200	47.9 s	1.13x
300	64.2 s	1.52x
400	71.5 s	1.69x

Side by side, shared sizes

jobSize	local (24c)	remote (4c)
50	49.1 s	39.3 s
100	44.7 s	34.9 s
150	49.8 s	35.6 s
300	64.2 s	36.2 s

Notes:

• On 24 cores, wall time climbs past job size ~250 (300 is 1.5x the fastest, 400 is 1.7x) as large chunks starve cores and the last worker drags the tail. The sweet spot sits around 60-130.
• On 4 cores the curve is flat from 100 upward, because with few workers the chunk size matters much less.
• The local box was under concurrent load during the sweep, so its absolute numbers are noisier and higher than the runner here; the shape of the curve is the point, not the absolute seconds.