Function validate_submission 

pub fn validate_submission(
    raw: &RawSubmission,
    tile_res: u32,
) -> Result<Submission, SubmitError>

Validate and convert a raw submit_chunk payload.

Non-finite heights (a NaN from a bad division, an inf from a failed fetch) are sanitized to 0.0: a NaN reaching LOD selection poisons it in a way that is near-impossible to trace back from the symptom.

An EMPTY normals means “finite-difference it for me” and yields normal: None; any other length mismatch is an error.