Fix target reference rest for A-pose retargeting

[snkmcb]

Summary

• Add an optional target reference rest in target joint order, separate from the UsdSkel rest used by undriven joints.
• Use the reference rotations in rest pose correction while preserving the existing three-argument API and default behavior.
• Expose canonical T-pose directions and shortest-rotation aim for format adapters, with regression coverage and updated documentation.

Fixes #32

Type of change

• Bug fix
• Feature or behavior change
• Documentation or test change
• Build or maintenance change

Validation

• Full Windows workspace build against OpenUSD 26.08 succeeded.
• ctest --test-dir build/cy2026-windows-x86_64-py313-usd --output-on-failure -j 4: 54/54 passed, including the installed consumer test.
• git diff --check: passed.

Checklist

• The change is focused and ready for review.
• I added or updated tests when behavior changed.
• I updated the owning documentation when a public fact or capability changed.
• I did not add a motion capture, clip, avatar model or screenshot without redistribution permission.
• This pull request does not contain a security report.