Add simple test configuration for easy verification

- Sun + Earth (1 AU) + Mars (1.5 AU)
- Circular orbits in XY plane
- Easy to verify: Earth 365 day period, Mars 687 day period
- Minimal 3-body system for testing orbital mechanics

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

configs/test_simple.txt

@@ -0,0 +1,22 @@
+# Simple Test Configuration
+# Two planets in circular orbits for easy position verification
+#
+# Orbital periods (for verification):
+#   Earth: ~365 days
+#   Mars: ~687 days
+#
+# At t=0:
+#   Earth starts at (1.0 AU, 0, 0) = (1.496e11 m, 0, 0)
+#   Mars starts at (1.5 AU, 0, 0) = (2.244e11 m, 0, 0)
+#   Both orbit counter-clockwise when viewed from +Z
+#
+# Format: name mass(kg) radius(m) x(m) y(m) z(m) parent_index r g b
+
+# Sun at origin (index 0)
+Sun 1.989e30 6.96e8 0 0 0 -1 1.0 1.0 0.0
+
+# Earth at 1 AU on positive X axis (index 1, parent Sun)
+Earth 5.972e24 6.371e6 1.496e11 0 0 0 0.0 0.5 1.0
+
+# Mars at 1.5 AU on positive X axis (index 2, parent Sun)
+Mars 6.39e23 3.3895e6 2.244e11 0 0 0 0.8 0.3 0.1