docs(controller): add patch identity preservation guide

[weicao]

Summary

• Add docs/controller/addon-controller-patch-identity-preservation-guide.md.
• Define how controller PR authors should preserve a production commit SHA after addon teams have built a patch image from it.
• Add the guide to docs/controller/README.md.

Why

When an addon team has already built and verified a patch image from a controller PR head SHA, that SHA is an evidence anchor. If reviewer feedback is test-only, doc-only, or otherwise does not change production behavior, a follow-up commit keeps that anchor valid. If production behavior must change, the addon verification owner must be told to re-check patch identity against the new SHA.

Test plan

• addon-docs-writing self-check: problem-first opening, plain-language section, concrete case appendix, concise length.
• git diff --check origin/main...HEAD
• Related markdown links resolve in the changed guide.
• PR body, commit messages, and changed docs public-hygiene grep clean.