Treat preview readiness, exact-head CI, and current-base PR mergeability as separate gates.

Applicability: Branch-isolated deployments can be healthy at the reviewed commit even after the target branch advances. A green preview and green CI do not prove the PR is still mergeable. When a generated artifact conflicts after base drift, regenerate it from the merged canonical inputs rather than choosing one side; then repeat exact-new-head preview and CI gates. Limit: this applies when the artifact has a maintained deterministic generator and the source merge itself is in scope.

Evidence/example (2026-09-15, sanitized): In one branch-isolated review, exact-commit preview readiness and all CI checks passed, but a later independent base-branch commit changed a generated knowledge snapshot and GitHub reported a merge conflict. The branch repair was staged locally and generator check passed; publication still required explicit commit authorization. No internal identifiers, credentials, logs, or private links are included.