[devmotion]

Tangents returned by @grad pullbacks now use the ChainRulesCore types throughout:

• nothing returned by a @grad pullback is converted to NoTangent() right after the pullback (and after _check_tangents), so _add_to_deriv! only handles AbstractZero, thunks and plain values.
• The internal fill rule returns NoTangent() for the dimensions.
• The @grad docstring lists the accepted tangents.

Existing pullbacks returning nothing keep working. The internal deriv buffers are unchanged (dense and preallocated).

Adds a test for a @grad pullback returning NoTangent(); the existing nothing tests cover the conversion.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.78%. Comparing base (fdf1b3d) to head (facd8ef).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #332   +/-   ##
=======================================
  Coverage   94.77%   94.78%           
=======================================
  Files          19       19           
  Lines        1703     1706    +3     
=======================================
+ Hits         1614     1617    +3     
  Misses         89       89           

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