Document new math.sum function design

function-math-sum.md

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+# math.sum
+
+## Summary
+
+``` math.sum(...: number): number ```
+
+Add a ```sum``` function to the ```math``` library, which adds the numbers together.
+
+## Motivation
+
+Numerical addition is one of the most fundamental mathematical operations. 
+Summing values is common in game development for damage calculations, currency totals, and statistical computations.
+
+A variadic function is particularly useful when the number of values to sum is not known in advance.
+
+A native implementation would improve ergonomics and could also enable implementation-specific optimizations.
+## Design
+
+Requesting a new function, ```math.sum```:
+
+```lua
+
+function math.sum(...: number) : number
+  local total = 0
+
+  for _, num in {...} do
+    total += num
+  end
+
+  return total
+end
+```
+
+## Drawbacks
+
+This introduces another function to the standard library.
+
+Additionally, any performance improvements may not be significant enough to justify the API expansion.
+
+## Alternatives
+
+Continue relying on user-defined helper functions.
+
+While this is straightforward, it results in repeated implementations across projects and misses the opportunity to provide a standard, 
+discoverable API for a common operation.