docs(country-map): SIP draft for Country Map plugin redesign

Living document tracking the design for the Country Map rework. Will
be filed as a proper SIP issue when the POC matures; this file is
working scratch in the meantime.

Captured so far:
- Motivation: disputed-borders flashpoints, Jupyter notebook pain,
  legacy explore_json endpoint
- Goals: configurable worldview, Admin 0 + Admin 1, per-deployment
  + per-chart customization, mapshaper-based reproducible build
  pipeline, modern chart/data endpoint
- Spike findings: NE UA worldview vs Default — confirms Crimea-as-
  Ukrainian + several other commonly-expected positions (Kosovo,
  Western Sahara, Palestine, Cyprus, Kashmir)
- Deprecation plan: in-UI "switch to new Country Map" button + banner
  on legacy, modeled on existing deprecated-chart pattern

Open questions tracked inline; implementation phases sketched.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>