fix: resolve SparseDistribution crash on zero/NaN probability mass

[Jonathangadeaharder]

Resolves #82

Changes

1. Fallback in SparseDistribution Constructor: Made SparseDistribution.__post_init__ fall back to a valid one-hot distribution on the first available token (greedy choice) if the sum of the probabilities is not finite or <= 0, rather than raising a crash-inducing ValueError.
2. Robust residual_distribution: Added not np.isfinite(total) checks to the fallback paths in residual_distribution (sparse, dense, and non-sparse branches) to prevent NaN values from bypassing target fallbacks.
3. Unit Tests: Added test cases in tests/test_sampling.py verifying fallback recovery on zero/NaN probability mass, as well as a mock test for residual_distribution NaN robustness.

All tests pass successfully.

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