[WIP] Optimize unit conversion operations to reduce .NET interop calls

[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

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This section details on the original issue you should resolve

<issue_title>Optimize unit conversion operations to reduce .NET interop calls</issue_title>
<issue_description>## Issue Overview

Reduce multiple .NET calls per unit conversion operation. Currently up to 4 .NET calls per toUnit() invocation.

Current Problem

File: R/utilities-units.R:148-242

Benchmark evidence shows significant overhead:

• 70k ms for 5000 iterations with 100k values
• Memory usage: 14,753 MB indicating heavy allocations
• Existing comments show "New version: 7k ms, 2290 mb" vs old "40k ms, 10556 mb" - indicating 82% improvement is possible

toUnit <- function(
  quantityOrDimension,
  values,
  targetUnit,
  sourceUnit = NULL,
  molWeight = NULL,
  molWeightUnit = NULL
) {
  # Up to 4 separate .NET calls:
  # 1. Convert molWeight to base unit
  # 2. Convert values to base unit
  # 3. Convert to target unit
  # Plus unnecessary as.numeric(c(values)) creating copies
}

Proposed Solution

1. Add early returns for same-unit conversions
2. Eliminate unnecessary as.numeric(c(values)) copies
3. Check if source and target are the same BEFORE any .NET calls
4. Combine multiple .NET calls where possible

toUnit <- function(
  quantityOrDimension,
  values,
  targetUnit,
  sourceUnit = NULL,
  molWeight = NULL,
  molWeightUnit = NULL
) {
  # Early return for same unit without source
  if (is.null(sourceUnit) && targetUnit == baseUnit) {
    return(values)
  }

  # Avoid unnecessary memory allocation if values are already numeric
  if (!is.numeric(values)) {
    values <- as.numeric(values)
  }

  # Check if source and target are the same BEFORE any .NET calls
  if (!is.null(sourceUnit) && sourceUnit == targetUnit) {
    return(values)
  }

  # Minimize .NET calls...
}

Expected Impact

• Priority: CRITICAL
• Estimated Impact: 50-80% reduction in unit conversion time (evidence from existing benchmark)
• Effort: Medium-High
• Testing: Run existing benchmark script tests/dev/script-benchmark-unitConversion.R to verify improvements

Evidence

From tests/dev/script-benchmark-unitConversion.R:

• Same unit conversion: 40+ ms per 1000 iterations
• Comments show 82% improvement is achievable with optimization

Parent Issue

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

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Parent Issue: #1765</issue_description>

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

• Fixes Optimize unit conversion operations to reduce .NET interop calls #1768