Defensive copy SET_STRING in getValue/populateFromEntries

## Description

`getValue()` returns live mutable set references. `populateFromEntries()` stores them directly. External code can mutate statistics bypassing events and dirty tracking.

## Solution

Copy set values on ingress, return `Collections.unmodifiableSet(...)` from `getValue()`.

## Files
- `PlayerStatisticData.java`

**Priority:** Medium