docs: add Laplace plane data limitations and decision note

docs/planetary_data.md

@@ -61,3 +61,15 @@ To get the true anomaly ν (which the TOML `orbit.true_anomaly` expects), solve
     tan(ν/2) = √((1+e)/(1−e)) · tan(E/2)
 
 Once ν is computed for each body, set it as `true_anomaly` in the config. The engine will then propagate from the J2000 snapshot forward.
+
+### Laplace Plane Data Limitations
+
+Reliable, authoritative data for the Laplace plane parameters (pole R.A./Dec. and tilt relative to each planet's equator) is difficult to find in standard planetary data sources. JPL Horizons and the JPL orbits page provide moon orbital elements relative to the Laplace plane but do not publish the Laplace plane's own orientation in ICRF.
+
+**Decision**: Use the planet's equatorial frame for moon orbital elements instead of converting from the Laplace plane. The Laplace plane is very close to the equatorial plane — tilted by only ~1° for Jupiter and ~0.3° for Saturn — so the resulting errors are negligible:
+
+- Inclination offset: ~0.3–1°
+- Node and periapsis offset: similar small amounts
+- Angular position error in space: ~0.3–1°
+
+This error is smaller than the uncertainties from using mean orbital elements (which ignore perturbations and resonances) and has no practical impact for simulation purposes.