Add Private Eval Layer and scorer contract docs

[t3chn]

Summary:

• Add Private Eval Layer documentation, private bundle manifest shape, visibility matrix, and canary guidance.
• Add scorer type contracts and decision-grade graduation criteria.
• Add reporting and redacted feedback rules, plus cross-links from README and authoring/comparison docs.

Issue:

• Supersedes Add Private Eval Layer and scorer contract docs #9 after Make consumer boundary product-agnostic #8 was merged and the stacked base branch was deleted.

Test plan:

• make validate
• make test
• make leak-check
• python3 -m ruff check .
• git diff --check