Issue #3627019: Refresh the codebase metrics page for the next alpha

Closes #3627019.

docs/metrics.md regenerated with php scripts/generate-metrics.php on 1.x at 5a2ac95c, cloc 2.08, dated 2026-09-30: 1,348 files and 121,718 lines of code, where the page said 1,292 and 116,885 (measured 2026-09-23 for 1.0.0-alpha18). A second run reproduces the page exactly, which is what the metrics job asserts on a tag; it also runs on this MR, since the MR touches the page. mkdocs build --strict passes. No other change.