Refresh the codebase metrics page for the next release

Closes #3629021.

docs/metrics.md regenerated with php scripts/generate-metrics.php on 1.x at fdffe784, with cloc 2.08: 1,401 files and 133,634 lines of code, against 1,348 and 121,718 on the page measured 2026-09-30. Re-measuring with METRICS_DATE=2026-10-07 reproduces the page byte for byte, which is what the metrics job checks. No other change.

Merge it last, after anything else meant for the release: any later commit on 1.x makes the page stale again.

AI-Generated: Yes (Claude Code was used to regenerate the page and write this description. I reviewed them before posting.)