ci(docs): record the -n 8 worker-count trial, which is worse on every leg

[wshallwshall]

Comment-only change to ci.yml. No functional change -- the three pytest_workers matrix values are untouched.

Why record a negative result

The argument for raising the xdist worker count is genuinely tempting, and someone will reach for it again: this suite is process-spawn heavy, so a worker blocked waiting on a pwsh or git child is not holding a core, which reads as 4 workers under-subscribing a 4-vCPU runner.

Measured, it is wrong -- worse on every leg and red on two of three.

leg	-n 4 baseline (n=2)	-n 8 (n=1)	delta	-n 8 outcome
ubuntu-latest	630 / 633s	731s	+16%	red, 2 timing assertions
windows-2022	967 / 1004s	1067s	+8%	green
windows-2025	1084 / 1092s	1366s	+25%	red, 3 tests

The -n 4 baselines are two runs agreeing within 4 percent: PR #411 and its post-merge push to main.

The failures are the mechanism, not noise

They cluster on exactly the tests that measure elapsed time or bind real ports, which is what contention degrades first:

• ubuntu lost test_outbound_batch.py's pacing floor and a super-linear-scaling ratio assertion.
• windows-2025 lost connscale_smoke, multishard_smoke, and session_mail's claim-verdict test -- the three heaviest multi-process tests, the last being the slowest single test in the suite.

Raw data is cited in the comment (trial branch claude/ci-worker-count-trial at 2bb733915, run 31968796353) so it outlives the branch, which is being deleted.