vision: port dag-gitgraph base lens

[Zychs]

Vision

DAG / gitgraph as a base lens over the same RepoSnapshot SSOT (ARCHITECTURE.md multi-lens platform).

Reality

src/lenses/dag-gitgraph.js STUB — throws on use. Port functions named in ARCHITECTURE/CATALOG from index.html harvest.

Exit

Lens picker offers spatial | radial | dag; renderer-contract compliance; no second snapshot truth.

Refs

Phase E · docs/renderer-contract.md · CATALOG disposition rows