docs(plan): accurate-analytics plan + public/private-carts design & plan

Analytics plan: 6 tasks (visitor_hash migration, hash/IP helpers, redirect
owner-exclusion + hashing, API self-view exclusion, reach plumbing+display).
Public/private carts (drafted by a background agent): design + plan, depends on
the analytics viewer-resolution plumbing landing first; uses migration 0007.

Author: mayur-tolexo