feat(core): add validated experiment domain models

[Staaaaaaaaar]

Summary

• introduce the ROS-independent rh_core package with immutable Experiment/Episode/PointNav models
• add strict schema-versioned YAML loading, structured validation errors, lifecycle guards, and deterministic termination priority
• define a full 3D Episode initial pose and a 3D PointNav goal, with an MVP configuration example
• add core-to-interface contract tests and extend repository lint/build coverage

Interface alignment

• remove start from PointNavTask and represent its 3D goal with geometry_msgs/PointStamped
• rename ResetEnv.start to initial_pose while retaining geometry_msgs/PoseStamped
• define PointNav success as 3D Euclidean distance from the configured robot tracking point

Validation

• make dev-check
• 4 colcon packages built successfully
• 102 tests, 0 errors, 0 failures, 0 skipped