refactor(atompub): remove unused categories renderer

[mdorman]

Summary

• remove the unreachable out-of-line AtomPub Categories Document renderer and public module surface
• preserve and strengthen coverage for inline Collection category discovery in the Service Document
• record the inline-only protocol boundary in a proposed ADR and the architecture view

Verification

• cargo xtask test-local -- -p common
• cargo xtask precommit through each commit hook
• cargo xtask prepush through the push hook
• whole-branch Standards and Spec reviews: no findings after the ADR correction

Closes #928