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 98.03922% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.18%. Comparing base (7e05098) to head (216786f).
⚠️ Report is 35 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/phase_transition/texture/kernels.py	98.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #926      +/-   ##
==========================================
+ Coverage   72.61%   73.18%   +0.57%     
==========================================
  Files         142      145       +3     
  Lines       21941    22640     +699     
==========================================
+ Hits        15933    16570     +637     
- Misses       6008     6070      +62     

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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.

[psavery]

I added a few more tests, and made a small API change to the symmetry-reduced misorientation so it runs much faster - it now uses the pre-existing rotations.misorientation in hexrd for batched, symmetry-aware misorientation instead of a per-pair loop (~40x faster on cubic symmetry). The one behavioral note: with symmetry enabled, one of the two operands must be a single reference orientation (it raises otherwise), which matches how the kernel is actually evaluated.

I'm rebasing #939 and #941 onto these changes, so those don't need fixing on your end. This PR looks good to me!

[psavery]

@bnmajor merge when ready!