test(fspy-benchmark): add a contended access row

[wan9chi]

Motivation

The benchmark's thread count was one constant shared by every suite, fixed at two. Two threads represent a normal tracked process, but they barely make writers fight over the IPC channel's counters, and every recorded access costs two atomic read-modify-writes on words every other thread is touching. Changes to how a record is claimed can therefore look free here while costing real time under a parallel build.

The count moves onto the suite, so the existing rows keep their two threads and stay comparable with earlier runs, and a new access-contended row runs the same opens under eight. It halves the iterations over half the opens, so four times the threads cost about the same wall clock.

Split out of #675, where it was used to measure claim-path changes. Replaces #679, which GitHub auto-closed and deleted the branch of when its base briefly absorbed this commit.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.21%  [-16.66% .. +25.37%]  overhead   +57.85%
dynamic/access             change  +2.61%  [ -8.75% .. +18.55%]  overhead    +5.54%
dynamic/access-relative    change  +0.10%  [ -6.79% ..  +7.34%]  overhead   +39.61%
dynamic/access-contended   change  +3.48%  [-19.04% .. +17.04%]  overhead   +16.26%
static/launch              change  +4.08%  [-18.08% .. +31.30%]  overhead  +153.07%
static/access              change  +1.89%  [ -7.93% .. +13.34%]  overhead  +940.83%
static/access-relative     change  +0.02%  [ -4.49% ..  +4.64%]  overhead +1317.31%
static/access-contended    change  -0.35%  [ -8.97% ..  +6.53%]  overhead +2349.63%

macos

dynamic/launch             change  -0.03%  [ -4.61% ..  +4.68%]  overhead  +231.74%
dynamic/access             change  -0.70%  [ -5.94% ..  +3.38%]  overhead    +2.89%
dynamic/access-relative    change  +1.10%  [ -7.76% ..  +7.00%]  overhead  +247.56%
dynamic/access-contended   change  +0.74%  [-82.51% .. +653.29%]  overhead    +2.99%

windows

dynamic/launch             change  -0.05%  [ -5.17% ..  +3.82%]  overhead   +29.44%
dynamic/access             change  +0.00%  [ -0.75% ..  +0.75%]  overhead    +1.32%
dynamic/access-relative    change  -0.18%  [ -1.44% ..  +1.47%]  overhead    +1.49%
dynamic/access-contended   change  +0.00%  [ -1.56% ..  +2.15%]  overhead    +1.81%