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.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.23%  [ -2.45% ..  +3.19%]  overhead   +48.79%
dynamic/access             change  -0.13%  [ -0.83% ..  +0.53%]  overhead    +5.65%
dynamic/access-relative    change  +0.09%  [ -0.72% ..  +0.72%]  overhead   +45.74%
dynamic/access-contended   change  +0.65%  [ -1.53% ..  +3.34%]  overhead    +8.77%
static/launch              change  +0.45%  [ -3.87% ..  +5.78%]  overhead  +143.92%
static/access              change  +0.13%  [ -0.61% ..  +0.93%]  overhead  +808.73%
static/access-relative     change  -0.10%  [ -1.06% ..  +0.91%]  overhead +1309.98%
static/access-contended    change  +0.30%  [ -0.54% ..  +1.87%]  overhead +3500.24%

macos

dynamic/launch             change  +0.23%  [ -2.83% ..  +4.01%]  overhead  +221.75%
dynamic/access             change  +0.00%  [ -2.19% ..  +2.72%]  overhead    +4.52%
dynamic/access-relative    change  +0.38%  [ -4.42% ..  +5.66%]  overhead  +260.34%
dynamic/access-contended   change  -0.09%  [ -7.01% ..  +4.77%]  overhead    +2.88%

windows

dynamic/launch             change  -0.30%  [-11.46% ..  +9.86%]  overhead   +25.72%
dynamic/access             change  +0.33%  [ -2.54% ..  +3.09%]  overhead    +1.40%
dynamic/access-relative    change  -0.18%  [ -2.33% ..  +3.49%]  overhead    +1.11%
dynamic/access-contended   change  +0.52%  [ -1.71% .. +15.69%]  overhead    +2.51%