Support dartpy with nanobind 3

[jslee02]

Summary

• Add nanobind 3 compatibility to dartpy source and isolated wheel builds while retaining nanobind 2 as the normal development lane.

Motivation / Problem

• Update pixi lockfile #3448 temporarily constrained isolated builds to nanobind 2 so Python 3.14 wheels could recover. This follow-up implements the durable cross-major compatibility needed to remove that temporary restriction.

Changes / Key Changes

• Accept nanobind 2.9.2 through 3.x in PEP 517 and CMake builds while keeping the default Pixi environment on nanobind 2.
• Isolate version-sensitive cast flags, type get/put calls, lifetime handling, and trampoline differences in a small compatibility header, using public nanobind APIs where available.
• Add polymorphic-cast and installed-wheel lifetime/trampoline regression coverage, plus compatibility-lane documentation and changelog notes.

Testing

• pixi run lint
• pixi run test-all (nanobind 2.14.0)
• pixi run -e cuda test-all
• pixi run -e py314-wheel wheel-build (isolated build resolved nanobind 3.0.0)
• pixi run -e py314-wheel wheel-verify
• pixi run -e py314-wheel wheel-test
• Built wheel SHA256: f9bf1363ec88f9501e1d2c3d0bcd0e8b37946f43e4401de9d36ffa1d2d557c1f
• Audited the declared minimum against nanobind 2.9.2 headers.
• Local wheel-repair cannot relabel this host-built artifact because the host emits newer glibc/GCC symbols; the hosted wheel workflow is the authoritative manylinux repair check.

Breaking Changes

• None

Related Issues / PRs (backports)

• Follow-up to Update pixi lockfile #3448

---

Checklist

• Milestone set (DART 7.0 for main, branch-matching DART 6.x release milestone for the active DART 6 LTS branch)
• CHANGELOG.md updated per docs/onboarding/changelog.md if required
• Add unit tests for new functionality
• Document new methods and classes
• Add Python bindings (dartpy) if applicable

[jslee02]

@codex review

[chatgpt-codex-connector]

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

Reviewed commit: 4891868916

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.67%. Comparing base (6d02cd1) to head (4891868).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3452   +/-   ##
=======================================
  Coverage   81.67%   81.67%           
=======================================
  Files         667      667           
  Lines       92082    92082           
=======================================
+ Hits        75209    75210    +1     
+ Misses      16873    16872    -1     

Flag	Coverage Δ
unittests	81.67% <ø> (+<0.01%)	⬆️

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