Add S2CondS2GridDistribution: conditional distribution on S²×S²

[Copilot]

Python port of the MATLAB S2CondS2GridDistribution — a conditional distribution f(a|b) over the Cartesian product of two 2-spheres, represented by grid values.

New class: S2CondS2GridDistribution

• Subclasses SdCondSdGridDistribution, restricting the grid to embedding dimension 3 (grid.shape == (n_points, 3))
• Overrides fix_dim and marginalize_out to return SphericalGridDistribution instead of the generic HypersphericalGridDistribution
• from_function factory delegates to the parent with dim=6 (S²×S²) and wraps the result

# VMF-based transition: f(xkk | xk) normalized per conditioning point
def trans(xkk, xk):
    result = np.zeros((xkk.shape[0], xk.shape[0]))
    for i in range(xk.shape[0]):
        result[:, i] = VonMisesFisherDistribution(xk[i], 1.0).pdf(xkk)
    return result

s2s2 = S2CondS2GridDistribution.from_function(trans, 112, fun_does_cartesian_product=True)

# Slice at a conditioning point → SphericalGridDistribution
sgd = s2s2.fix_dim(2, s2s2.grid[0])

Tests

Mirrors the MATLAB S2CondS2GridDistributionTest suite:

• Warning-free construction from a normalized VMF transition
• Warning emitted for unnormalized transitions
• Warning-free construction from a manually normalized custom transition
• Cartesian-product and non-Cartesian-product factory modes produce identical results
• fix_dim(2, point).mean_direction() recovers the conditioning point
• fix_dim and marginalize_out return SphericalGridDistribution instances

[github-actions]

❌MegaLinter analysis: Error

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	7.87s
✅ JSON	prettier	2	0	0	0	0.43s
✅ JSON	v8r	2		0	0	2.38s
✅ MARKDOWN	markdownlint	1	0	0	0	0.66s
✅ MARKDOWN	markdown-table-formatter	1	0	0	0	0.24s
✅ PYTHON	bandit	252		0	0	3.03s
✅ PYTHON	black	252	1	0	0	4.47s
❌ PYTHON	flake8	252		2	0	1.76s
✅ PYTHON	isort	252	2	0	0	0.48s
✅ PYTHON	mypy	252		0	0	3.96s
❌ PYTHON	pylint	252		4	0	70.22s
❌ PYTHON	ruff	252	2	2	0	0.05s
✅ REPOSITORY	checkov	yes		no	no	21.56s
✅ REPOSITORY	gitleaks	yes		no	no	5.23s
✅ REPOSITORY	git_diff	yes		no	no	0.05s
✅ REPOSITORY	secretlint	yes		no	no	5.67s
✅ REPOSITORY	syft	yes		no	no	3.48s
✅ REPOSITORY	trivy-sbom	yes		no	no	1.7s
✅ REPOSITORY	trufflehog	yes		no	no	18.06s
✅ YAML	prettier	4	0	0	0	0.5s
✅ YAML	v8r	4		0	0	5.54s
✅ YAML	yamllint	4		0	0	0.44s

Detailed Issues

❌ PYTHON / flake8 - 2 errors

pyrecest/tests/distributions/test_s2_cond_s2_grid_distribution.py:99:13: F401 'numpy as np' imported but unused
pyrecest/tests/distributions/test_s2_cond_s2_grid_distribution.py:152:20: F821 undefined name 'np'

❌ PYTHON / pylint - 4 errors

************* Module pyrecest.tests.distributions.test_s2_cond_s2_grid_distribution
pyrecest/tests/distributions/test_s2_cond_s2_grid_distribution.py:7:0: W0622: Redefining built-in 'sum' (redefined-builtin)
pyrecest/tests/distributions/test_s2_cond_s2_grid_distribution.py:99:12: W0611: Unused numpy imported as np (unused-import)
pyrecest/tests/distributions/test_s2_cond_s2_grid_distribution.py:152:19: E0602: Undefined variable 'np' (undefined-variable)
pyrecest/tests/distributions/test_s2_cond_s2_grid_distribution.py:154:26: W0613: Unused argument 'xk' (unused-argument)

-----------------------------------
Your code has been rated at 9.99/10

❌ PYTHON / ruff - 2 errors

F821 Undefined name `np`
   --> pyrecest/tests/distributions/test_s2_cond_s2_grid_distribution.py:150:20
    |
148 |         def f_trans1(xkk, xk):
149 |             vals = dist.pdf(xkk)  # (n1,)
150 |             return np.tile(vals[:, None], (1, xk.shape[0]))  # (n1, n2)
    |                    ^^
151 |
152 |         def f_trans2(xkk, xk):
    |

Found 2 errors (1 fixed, 1 remaining).

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[github-actions]

Test Results

0 files   -    12  0 suites   - 12   0s ⏱️ - 3h 18m 28s
0 tests  -   571  0 ✅  -   571  0 💤 ±    0  0 ❌ ±0 
0 runs   - 7 064  0 ✅  - 5 880  0 💤  - 1 184  0 ❌ ±0 

Results for commit 5d5deff. ± Comparison against base commit 8d19d73.

♻️ This comment has been updated with latest results.