Merge branch 'adr/unified-rendering-engine'

Adds ADR-702 (Proposed): one r3f engine for all graph viz, with
projection / node-mode / physics-backend as internal axes; edges stay
line/curve with bezier; 10k-node target; distance-culled Html labels;
uniform instancedMesh picking across sprite/poly.

Spike under spike/unified-3d/ validated kg data shape compatibility
against the atlassian-graph reference stack (headless + browser session
on 2026-04-20 confirmed GPU physics, selection caret, palette ramps, edge
types to detail panel). Four findings — edge-type coloring, directed
arrows, id/label separation, palette as opaque function — all propagated
into the engine contract.

Author: aaronsb