Add distro build toggles

[jslee02]

• Add DART_BUILD_TESTS, DART_BUILD_EXAMPLES, and DART_BUILD_TUTORIALS toggles so packagers can disable heavy targets and gate them on dart-gui-osg availability.
• Propagate the knobs through pixi run workflows plus docs/changelog so CI and local builds stay consistent.
• Fix the pkg-config Cflags export to report the installed include path for downstream consumers.

Tested with CMAKE_BUILD_PARALLEL_LEVEL=12 pixi run lint.

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Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes
• Add unit tests for new functionality
• Document new methods and classes
• Add Python bindings (dartpy) if applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.27%. Comparing base (ad5a99f) to head (6715a63).
⚠️ Report is 346 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2177   +/-   ##
=======================================
  Coverage   62.26%   62.27%           
=======================================
  Files         393      393           
  Lines       33207    33207           
  Branches     4137     4138    +1     
=======================================
+ Hits        20676    20679    +3     
+ Misses      12531    12528    -3     

Flag	Coverage Δ
unittests	62.27% <ø> (+<0.01%)	⬆️

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see 1 file with indirect coverage changes

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[jslee02]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

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