docs(compat): record verified CE v1.3.1 API matrix and 404 disambiguation

Probed local self-host instance: core CRUD endpoints OK (both /work-items/
and legacy /issues/); pages, issue-types, initiatives, estimates and
features APIs return 404 (missing endpoint). Documented the JSON body
difference between missing-endpoint and missing-resource 404s for the
planned Stage 5 compatibility layer. Roadmap: Stage 1 catalog, Stage 6 PAT
primary mode and E12.1 pages verification checked off.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>