docs: distinguish macOS P95 calibration data (PR #508)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: karthiknadig

docs/QUALITY_SNAPSHOTS.md

@@ -21,7 +21,7 @@ A metric blocks when it exceeds both its absolute and relative budget:
 | Time to first environment P50 | 20 ms / 100% | 25 ms / 50% | 150 ms / 50% |
 | Time to first environment P95 | 250 ms / 100% | 500 ms / 100% | 750 ms / 100% |
 
-Each cell is `absolute / relative`. The budgets reflect observed GitHub-hosted runner variance from 11 consecutive main-branch baselines. Tighten them when a noisy path is fixed rather than normalizing a known regression into the baseline.
+Each cell is `absolute / relative`. The Linux and Windows budgets plus the macOS P50 budgets reflect observed GitHub-hosted runner variance from 11 consecutive main-branch baselines. Tighten them when a noisy path is fixed rather than normalizing a known regression into the baseline.
 
 The macOS P95 budgets were recalibrated after PR #506 (tracking issue #504) using three unchanged-content pull-request runs and the exact merged baseline at `f0c62d9`. Their absolute headroom is four to six times the observed post-fix run-to-run range.