feat(chart): add bubble chart support

[PrzemyslawKlys]

Summary

• add ChartBubble definition and chart rendering logic
• expose New-ImageChartBubble cmdlet
• document usage and test bubble chart generation

Testing

• dotnet build Sources/ImagePlayground.sln
• pwsh -NoLogo -Command ./ImagePlayground.Tests.ps1

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https://chatgpt.com/codex/tasks/task_e_689c290307c8832e8e0e9632ce7ecd65

[codecov]

Codecov Report

❌ Patch coverage is 51.89873% with 76 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.12%. Comparing base (c1a5436) to head (6b0322b).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
Sources/ImagePlayground.Chart/Charts.cs	55.40%	53 Missing and 13 partials ⚠️
Sources/ImagePlayground.Chart/ChartBubble.cs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
- Coverage   66.30%   65.12%   -1.18%     
==========================================
  Files          46       48       +2     
  Lines        1653     1689      +36     
  Branches      238      244       +6     
==========================================
+ Hits         1096     1100       +4     
- Misses        463      497      +34     
+ Partials       94       92       -2     

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