CI test-scope detection diffed changed files with a two-dot
`git diff --name-only base head`, where base is the moving tip of `next`.
A PR branch cut from a slightly older `next` surfaced every product file
`next` had gained since the merge-base, flipping product_changed/full_matrix
and running the full Windows/macOS matrix + coverage on docs-only PRs.
Switch to a three-dot `git diff --name-only base...head` (vs the merge-base),
matching GitHub's PR "Files changed" semantics. Add a regression test that
builds a stale-base topology, plus a guard test pinning `fetch-depth: 0` on
the `changes` job (required for the merge-base to be locally available).
Closes#837
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
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