[ariwired]

Closes #8

Stacked on #10 (base is the engine branch; retarget to backend after #10 merges).

Coverage

• LEFT_OF / RIGHT_OF / ABOVE / BELOW, including equal centers and margin boundary
• NEAR: exactly at threshold (inclusive), slightly outside, diagonal gap, configurable
• OVERLAPS: partial, containment, touching edges/corner (no overlap), ratio boundary
• Invalid input: zero-area, negative dimensions, NaN/inf, non-numeric coordinates, self-relations, duplicate ids, invalid config
• Reusable fixtures; no Neo4j required; deterministic