Remove top-level engineering directory

[jcwal1516]

Supersedes #76, which GitHub automatically closed when the head branch was renamed to the repository's configured branch/ namespace.

Summary

• remove the generic top-level engineering/ documentation drawer
• delete completed audits, drafts, and repository-policy prose instead of relocating it
• remove the isolated 0.7.1 digital-pathology audit and historical benchmark diagnostic diaries
• keep live 0.8.1 public-API evidence under docs/release-evidence/public-api/
• reconcile canonical release/security/API documentation with published v0.8.1
• make static site pages link to the canonical release policy and crates.io instead of duplicating mutable version numbers
• update CI planning, semver checks, release docs, and repository-lint coverage for the new ownership

LLM guidance audit

The historical Fast Path For LLM-Assisted Use README section was already removed in d8e89e9e. This PR does not restore it. Current HEAD has no AGENTS, Claude, Copilot, prompt, or other LLM-specific instruction file.

Verification

• cargo test -p xtask --test repo_lint — 49 passed
• cargo xtask release-integrity — passed
• cargo xtask public-support --final — passed
• cargo xtask doc — passed
• cargo clippy -p xtask --test repo_lint --all-features -- -D warnings — passed
• python3 -m unittest discover -s scripts/tests -p 'test_*.py' -v — 78 passed
• cargo fmt --all -- --check — passed
• typos README.md CHANGELOG.md CONTRIBUTING.md SECURITY.md CODE_OF_CONDUCT.md NOTICES.md docs corpus crates --format brief — passed
• local Markdown/HTML link scan — 0 broken local links
• git diff --check — passed
• exact-SHA quick Metal validation — passed in 3m54s
• exact-SHA quick CUDA validation — passed in 10m1s
• GitHub PR required checks — passed