docs(test): add PR backport and DaemonSet narrow rollout guides

[weicao]

Summary

• Add docs/test/addon-pr-vs-release-tag-validation-backport-guide.md: when a PR main image cannot run in an older release-tag runtime, use a local-only release-tag-base backport image for scoped validation.
• Add docs/test/addon-daemonset-narrow-node-image-rollout-guide.md: validate an unpublished DaemonSet image on one target node with OnDelete, local sideload, and explicit revert.
• Add docs/cases/sqlserver/sqlserver-pitr-apelocal-nodeunpublish-lazy-fallback-case.md: SQL Server PITR / ape-local lazy-unmount fallback case with scoped evidence and non-release-ready boundary.
• Update docs/test/README.md index.

Review notes

• Method guides are engine-neutral and under 150 lines.
• The SQL Server case is engine-specific and under 120 lines.
• The case keeps only the minimum evidence needed for backport, narrow DaemonSet rollout, runtime caveats, injected-holder closure, and cleanup boundary.

Validation

• git diff --check origin/main..HEAD
• local markdown link check for changed docs
• commit body public-hygiene grep