docs(adr): add ADR-702 unified graph rendering engine + validation spike

Proposes replacing the three independent graph rendering stacks (d3.js 2D,
react-force-graph-3d wrapper, raw three.js embedding scatter) with a single
r3f + instanced + GPU-physics engine parameterized along projection, node
mode, and physics backend axes. Edges stay line/curve geometry in both
projections with bezier support from phase 1.

Spike under spike/unified-3d/ pipes real kg concept data through the
atlassian-graph reference stack to validate the shape compatibility.
Headless verification passes; visual verification pending a browser
session. Four design refinements surfaced from the spike are incorporated
into the ADR: edge coloring by edge-type, directed-edge arrows, separate
id/label fields, palette as opaque function.

Author: aaronsb