docs: add sim coordinate frame conventions (Fixes #53)

docs/coordinate_conventions.md

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+# Sim Coordinate Frame Conventions (Wave 2)
+
+## Overview
+
+This document describes the coordinate system conventions used in Lappa simulation fixtures and path planning examples.
+
+## Units
+
+- **Position**: meters (m)
+- **Angle**: radians (rad)
+- **Velocity**: meters per second (m/s)
+- **Angular velocity**: radians per second (rad/s)
+
+## Coordinate System
+
+- **Origin** (`0, 0`): World frame reference point
+- **X-axis**: Forward direction (robot heading)
+- **Y-axis**: Left direction (90° counter-clockwise from X)
+- **Z-axis**: Upward direction (right-hand rule)
+
+## Path Fixture Conventions
+
+All path fixtures follow these rules:
+
+1. Positions are in meters from world origin
+2. Angles are in radians, measured counter-clockwise from X-axis
+3. Velocities are instantaneous (per timestep)
+4. Negative Y values indicate rightward movement
+
+## Example
+
+```
+# Patrol loop fixture
+waypoint_1: {x: 2.0, y: 0.0, theta: 0.0}    # 2m forward
+waypoint_2: {x: 2.0, y: 2.0, theta: 1.57}   # 2m left (π/2 rad)
+waypoint_3: {x: 0.0, y: 2.0, theta: 3.14}   # 2m backward (π rad)
+waypoint_4: {x: 0.0, y: 0.0, theta: 4.71}   # back to origin (3π/2 rad)
+```
+
+## References
+
+- ROS 2 REP 103: Uniform Standard for Physical Robot Metadata
+- REP 105: Standard for Units Symbolism
+- Lappa main README: [link to main documentation]