[WIP] Replace rbind.data.frame with data.table for efficient row binding

[Claude]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Replace rbind.data.frame with data.table for efficient row binding</issue_title>
<issue_description>## Issue Overview

Replace inefficient rbind.data.frame operations with data.table::rbindlist for binding many small data frames in population simulation result processing.

Current Problem

File: R/utilities-simulation-results.R:94-97

allIndividualProperties <- do.call(
  rbind.data.frame,
  c(individualPropertiesCache, stringsAsFactors = FALSE)
)

Issues:

• rbind.data.frame is slow for binding many small data frames
• Creates intermediate copies during the binding process
• For 1000 individuals: combines 1000 separate list structures into one data frame
• Potential O(n²) complexity in worst case due to repeated memory allocations

Proposed Solution

Option 1: Use data.table for efficient row binding

library(data.table)
allIndividualProperties <- data.table::rbindlist(
  individualPropertiesCache,
  use.names = TRUE
)

Option 2: Pre-allocate and fill by columns

nRows <- length(individualIds) * valueLength
allIndividualProperties <- data.frame(
  IndividualId = integer(nRows),
  Time = numeric(nRows),
  stringsAsFactors = FALSE
)
# Pre-allocate columns for each covariate
for (covariateName in covariateNames) {
  allIndividualProperties[[covariateName]] <- numeric(nRows)
}
# Fill in values more efficiently
rowIdx <- 1
for (individualIndex in seq_along(individualIds)) {
  endIdx <- rowIdx + valueLength - 1
  allIndividualProperties$IndividualId[rowIdx:endIdx] <- individualIds[individualIndex]
  # ... fill other columns
  rowIdx <- endIdx + 1
}

Expected Impact

• Priority: HIGH
• Estimated Impact: 40-60% reduction in data frame construction time
• Effort: Low
• Testing: Benchmark with various population sizes (100, 1000, 10000 individuals)

Implementation Notes

• Affects all population simulation result processing
• Verify that data.table is available as a dependency or add it
• Ensure backward compatibility of output format

Parent Issue

This is part of the comprehensive performance optimization analysis in #1765

---

Parent Issue: #1765</issue_description>

Comments on the Issue (you are @claude[agent] in this section)

• Fixes Replace rbind.data.frame with data.table for efficient row binding #1769