Color picker - Lab format: use roundoff optional #13603 (#42986)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
The default CIELab format rounds the values to two decimal places, which
is a degree of precision that isn't always needed. This PR adds an
optional formatting character (i) to the three CIELab format parameters,
which rounds the value to the nearest integer.

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## PR Checklist

- [x] Closes: #13603/#14863. Note that in the discussion for #13603,
there are some additional suggestions that this PR doesn't address.
- [ ] **Communication:** Haven't gotten the green light for this
approach with the core contributors yet. Happy to pivot to a different
approach if needed.
- [ ] **Tests:** Added/updated and all pass
- [x] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

In the case where a or b get rounded to -0 (e.g. -0.0001 rounds to -0),
the negative sign gets removed. However, I noticed during testing that
the default format (rounding to two decimal places) retains the negative
sign in these situations (see third screenshot). I can a) revert to
keeping the -0 for the new rounding behavior, b) change -0 to 0 for
other rounded values, or c) leave it as-is. Also open to suggestions.

I can update the docs as well, if we're happy with the approach.

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
- Settings: Can change the default CIELab format to display rounded
values
- Settings: Can create a custom format with rounded CIELab values
- Color Picker: Rounded values are displayed when hovering and as a
saved color
<img width="1076" height="1520" alt="powertoys-cielab"
src="https://github.com/user-attachments/assets/8a67142d-d7f4-49bc-b1ba-ad9304235218"
/>
<img width="447" height="390" alt="powertoys-cielab-selected"
src="https://github.com/user-attachments/assets/c96d3bc9-cac7-4470-af3f-b2bce78d0915"
/>
<img width="445" height="389" alt="powertoys-cielab-selected-0"
src="https://github.com/user-attachments/assets/c329bc5b-c18a-4f61-a808-0fa5050e09ed"
/>

---------

Co-authored-by: vanzue <vanzue@outlook.com>

src/common/ManagedCommon/ColorFormatHelper.cs

@@ -515,8 +515,7 @@ namespace ManagedCommon
                     return lightnessL.ToString(CultureInfo.InvariantCulture);
                 case "Lc":
                     var (lightnessC, _, _) = ConvertToCIELABColor(color);
-                    lightnessC = Math.Round(lightnessC, 2);
-                    return lightnessC.ToString(CultureInfo.InvariantCulture);
+                    return ColorPercentFormatted(lightnessC, paramFormat, 2);
                 case "Lo":
                     var (lightnessO, _, _) = ConvertToOklabColor(color);
                     lightnessO = Math.Round(lightnessO, 2);
@@ -531,12 +530,10 @@ namespace ManagedCommon
                     return blackness.ToString(CultureInfo.InvariantCulture);
                 case "Ca":
                     var (_, chromaticityA, _) = ConvertToCIELABColor(color);
-                    chromaticityA = Math.Round(chromaticityA, 2);
-                    return chromaticityA.ToString(CultureInfo.InvariantCulture);
+                    return ColorPercentFormatted(chromaticityA, paramFormat, 2);
                 case "Cb":
                     var (_, _, chromaticityB) = ConvertToCIELABColor(color);
-                    chromaticityB = Math.Round(chromaticityB, 2);
-                    return chromaticityB.ToString(CultureInfo.InvariantCulture);
+                    return ColorPercentFormatted(chromaticityB, paramFormat, 2);
                 case "Oa":
                     var (_, chromaticityAOklab, _) = ConvertToOklabColor(color);
                     chromaticityAOklab = Math.Round(chromaticityAOklab, 2);
@@ -595,6 +592,24 @@ namespace ManagedCommon
             }
         }
 
+        private static string ColorPercentFormatted(double colorPercentValue, char paramFormat, int defaultDecimalDigits)
+        {
+            switch (paramFormat)
+            {
+                case 'i':
+                    double roundedColorPercentValue = Math.Round(colorPercentValue);
+                    if (roundedColorPercentValue == 0)
+                    {
+                        // convert -0 to 0
+                        roundedColorPercentValue = 0.0;
+                    }
+
+                    return roundedColorPercentValue.ToString(CultureInfo.InvariantCulture);
+                default:
+                    return Math.Round(colorPercentValue, defaultDecimalDigits).ToString(CultureInfo.InvariantCulture);
+            }
+        }
+
         public static string GetDefaultFormat(string formatName)
         {
             switch (formatName)