chore: restore earcut_impl as deprecated and document utils3d

[ciscorn]

• I agree to follow the project's code of conduct.
• I added an entry to the project's change log file if knowledge of this change could be valuable to users.
  • Usually called CHANGES.md or CHANGELOG.md
  • Prefix changelog entries for breaking changes with "BREAKING: "

[Copilot]

Pull request overview

Restores the previously-available earcut_impl entry point as a deprecated API while improving documentation around 3D→2D polygon projection helpers.

Changes:

• Added module/function-level rustdoc for utils3d and project3d_to_2d.
• Refactored Earcut::earcut to delegate to a new internal run method.
• Reintroduced Earcut::earcut_impl as a deprecated method calling the internal run.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/utils3d.rs	Adds module + function documentation for the 3D-to-2D projection helper.
src/lib.rs	Reintroduces earcut_impl as deprecated and extracts shared logic into run.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.56%. Comparing base (3f92ebd) to head (20a6089).

Files with missing lines	Patch %	Lines
src/lib.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
- Coverage   99.72%   99.56%   -0.17%     
==========================================
  Files           3        3              
  Lines        1851     1857       +6     
==========================================
+ Hits         1846     1849       +3     
- Misses          5        8       +3     

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