Add SO3 representation conversion support

[FlorianPfaff]

This PR is stacked on #1931 and adds SO(3) / SO(3)^K support to the representation-conversion gateway.

Main changes:

• adds SO(3)-aware aliases such as so3_dirac, so3_particles, so3_tangent_gaussian, so3_product_dirac, and so3_product_tangent_gaussian
• makes generic particles / dirac aliases resolve to SO(3)-specific Dirac representations for SO(3) and SO(3)^K sources
• makes generic gaussian resolve to tangent-Gaussian approximations for SO(3) / SO(3)^K sources
• adds so3_conversion.py with registered converters rather than modifying the SO(3) distribution classes directly
• supports SO(3) tangent-Gaussian -> SO(3) Dirac via sampling
• supports SO(3) Dirac -> SO(3) tangent Gaussian via local tangent-space moment matching
• supports SO(3)^K product tangent-Gaussian -> product Dirac via sampling
• supports SO(3)^K product Dirac -> product tangent Gaussian via local tangent-space moment matching
• adds tests for SO(3) and SO(3)^K conversions
• updates representation-conversion docs

Notes:

• This PR should be retargeted to main after Add representation conversion aliases #1931 is merged.
• The implementation keeps SO(3)-specific fitting code in a small registration module to avoid expanding the core conversion gateway too much.

[github-actions]

✅MegaLinter analysis: Success

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✅ REPOSITORY	syft	yes		no	no	3.44s
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[github-actions]

Test Results

    12 files  ± 0      12 suites  ±0   4h 54m 13s ⏱️ + 34m 54s
 1 398 tests + 5   1 398 ✅ + 5      0 💤 ±0  0 ❌ ±0 
17 236 runs  +60  13 648 ✅ +60  3 588 💤 ±0  0 ❌ ±0 

Results for commit c708d24. ± Comparison against base commit b8826cd.

♻️ This comment has been updated with latest results.