Add de la Vallée Poussin kernel on SO(3)

[bnmajor]

Overview

Adds a de la Vallée Poussin kernel implementation on SO(3) for constructing smooth orientation distribution functions.

New modules:

• SO3Kernel
• DeLaValleePoussinKernel

New public API:

• DeLaValleePoussinKernel(halfwidth: float) - constructor, halfwidth in radians
• .eval(R1, R2) - evaluate kernel between rotation matrices
• .eval_centered(R, center) - convenience wrapper for unimodal distributions
• .misorientation_angle(R1, R2) - compute misorientation angle from 3×3 rotation matrices

Mathematical basis:
The kernel is defined as K(ω) = C · cos(ω/2)^(2κ), where C is a Beta-function normalization constant and κ is derived analytically from the halfwidth via κ = ln(0.5) / (2·ln(cos(h/2))).

Affected Workflows

N/A

Documentation Changes

User-facing documentation for the texture module will be added in the module integration PR once the full API is finalized.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.48%. Comparing base (3029df0) to head (fd1dc01).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/texture/kernels.py	87.50%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #926      +/-   ##
==========================================
+ Coverage   71.44%   71.48%   +0.03%     
==========================================
  Files         142      143       +1     
  Lines       21935    21983      +48     
==========================================
+ Hits        15672    15714      +42     
- Misses       6263     6269       +6     

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[psavery]

To do:

• move the texture directory inside the phase_transition directory
• Add a simple test for running the __str__ and __repr__ functions
• Keep in mind that for misorientation_angle, we may need to take into account crystal symmetry (depending on how it will be used), and misorientation in the rotations.py file may be able to work.