ci(perf): add the weekly baseline tightening workflow (#1760)

docs/architecture/validation-map.md

@@ -260,7 +260,12 @@ bytes on the initial path (the J1 counter `renderer.initialBytes`).
 `tools/performance/journey-baselines.json`; baselines only move down. CI runs
 the same check in the `Initial bytes ratchet` job of `ci.yml` for PRs that
 target `master` and for `master` pushes (dispatch it with
-`gh workflow run ci.yml --ref <branch>` for a stacked branch). `perf:journeys` builds the `electron-performance` configuration and runs every
+`gh workflow run ci.yml --ref <branch>` for a stacked branch). The weekly
+`performance-ratchet.yml` workflow lowers baselines through a bot PR; validate
+a change to it with `gh workflow run performance-ratchet.yml --ref <branch>`,
+which measures but opens no PR off `master`. Dispatch needs the workflow file
+on `master`; before that, see the temporary-trigger note under Weekly
+tightening in the performance journeys document. `perf:journeys` builds the `electron-performance` configuration and runs every
 journey spec against the Xtream mock: J1 launch, then J2 open-source (a
 second set of launches, each followed by the click on the portal card), both
 written to the same summary file; its probe specs run with