Project: s41467-025-58868-2 | Function: classify.cell (r)

system: You are an expert R programmer implementing algorithms from scientific papers.
Complete the masked function based on the algorithm description.
Follow R best practices and maintain consistent code style.
Return **only** the complete R function code.

user: Complete this R function:

**DESCRIPTION:** Statistical cell classification using Mahalanobis distance to multiple populations. Implements probabilistic classification based on covariance structure of cell populations.

**KEY TERMS:** {
  "mah_dist": "Mahalanobis distances",
  "cell": "cell measurements",
  "threshold": "classification threshold"
}

**CONTEXT CODE:** # Minimal masked version for testing - contains only the masked classify.cell function

classify.cell <- function(cell, threshold) {
  stop("Function implementation has been masked")

Complete the function implementation replacing "stop("Not implemented")".