docs: rewrite README as KRL architecture introduction + add visual map

- Replace README with KRL stack overview: four operations (Construct,
  Transform, Resolve, Retrieve) and the TangleIR spine
- Add docs/krl_map.html — visual architecture map (canonical location,
  referenced from nextgen-languages, nextgen-databases, quandledb READMEs)
- Clarify that Tangle is the general-purpose host language; KRL is one
  DSL built on it, not Tangle's reason for existing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath