Add startup parameter validation with actionable error messages

[manankharwar]

What

Add a validation pass at node startup that checks for common misconfigurations and prints actionable error messages before the filter starts.

Why

The most common support questions are caused by misconfigured parameters: wrong topic names, invalid covariance values, IMU frame mismatches. Right now the filter silently behaves incorrectly. A startup check surfaces these immediately.

Scope

Checks to add (not exhaustive, use judgment):

• `gnss.base_noise_xy` and `gnss.base_noise_z` are positive
• `imu.topic` is non-empty
• Process noise values (`q_*`) are all positive
• `chi2_threshold` is within a sensible range (e.g. warn if below 3.0 or above 20.0)
• If `imu.remove_gravitational_acceleration: false`, warn that the IMU measurement function expects raw specific force

Each check should log a clear `RCLCPP_ERROR` or `RCLCPP_WARN` with the parameter name and what the valid range is.

Acceptance criteria

• Misconfigured parameter prints a clear error at startup, not a cryptic filter divergence 10 seconds in
• No existing tests broken
• New test covering at least one validation case

Relevant files

• `fusioncore_ros/src/fusioncore_node.cpp` (node startup)
• `fusioncore_ros/include/fusioncore_ros/params.hpp` (parameter declarations)