SignificantDigitsNumberRounder not rounding to Zero

### Describe the bug

SignificantDigitsNumberRounder does not round small values to zero

### Why is this important?

In a NumberBox values might show as 0.03 -> 0.02 -> 0.01 but then 0.000000xx when it should actually be 0.00

### Steps to reproduce the bug

In Visual Studio create a default WinUi Blank App (Packaged) C++
In a suitable place put the code
```
Windows::Globalization::NumberFormatting::SignificantDigitsNumberRounder sdnr;
sdnr.SignificantDigits(2);
double d = sdnr.RoundDouble(0.00001);
```
inspect the resulting value of d

### Actual behavior

resulting value is not rounded to 0.00

### Expected behavior

resulting value should be rounded to 0.00

### Screenshots

_No response_

### NuGet package version

None

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Start with the SignificantDigitsNumberRounder entry point and reproduce the supplied C++ WinUI example using SignificantDigits(2) and RoundDouble(0.00001). Trace the rounding behavior for small values and add regression coverage showing that the result becomes zero while preserving the expected behavior for ordinary values.

Written by the indexing model from the issue text.