add support code for polygon workflows

[jveitchmichaelis]

Description

This PR adds a number of support features for polygon modeling. While the exact models are under discussion, the additions in this PR are generic:

• Visualization
• Geometry format detection
• Metric support
• COCO RLE mask decoding
• Mask <> Polygon conversion
• Polygon NMS

AI-Assisted Development

Had Claude pluck this out from another branch and then make significant edits to the language + style, again.

AI tools used (if applicable):

Claude Opus 5.

[codecov]

Codecov Report

❌ Patch coverage is 89.38053% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.28%. Comparing base (2b26da4) to head (4b7faa4).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/deepforest/predict.py	68.00%	8 Missing ⚠️
src/deepforest/utilities.py	95.38%	3 Missing ⚠️
src/deepforest/visualize.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1419      +/-   ##
==========================================
+ Coverage   86.67%   87.28%   +0.60%     
==========================================
  Files          28       28              
  Lines        4069     4129      +60     
==========================================
+ Hits         3527     3604      +77     
+ Misses        542      525      -17     

Flag	Coverage Δ
unittests	87.28% <89.38%> (+0.60%)	⬆️

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[jveitchmichaelis]

Added some tests. Will put a separate PR in to add unit tests for predict.py as coverage is a bit scattered there.

[jveitchmichaelis]

Actions is currently degraded, so waiting to re-run

[bw4sz]

Great, this looks ready for main.