Extend CI path filtering so a scoped change does not pay for the full matrix

The `changes` job already skips runs that are irrelevant to a PR. Extend it so a docs-only change, or one confined to a single engine, does not trigger every lane.

Care is needed on two fronts: a shared-code change must still fan out, and a skipped job has to report a green context rather than nothing, or branch protection waits forever.

Part of #19524.

Start with the `changes` job and trace how its current relevance checks select matrix lanes. Verify the behavior for docs-only changes, single-engine changes, and shared-code changes, then confirm that skipped jobs still publish a green context so branch protection can complete.

Written by the indexing model from the issue text.