docs(designs): golden templates and semver check

PR 0 of the golden-templates rollout. Documents the corpus, the
in-process harness, the daily integration tier, and the semver gate
that requires a major-version bump when an existing pinned output
changes.

After #8637 shipped, four output-shape regressions slipped past test
coverage (#9004, #9005, #9027, #9029) before #9033 made local LE
processing opt-in. The design pins the expanded output of
representative templates so future regressions surface in code review
rather than after release.

No code changes; review and discussion happen on this PR before
infrastructure lands in PR 1.

Author: bnusunny