Add build_permuted_exploration_dataset for relational candidate selection, preserving intra-group relationships (between children under the same parent).

[copybara-service]

Add build_permuted_exploration_dataset for relational candidate selection, preserving intra-group relationships (between children under the same parent).

Implements the multi-slot permuted exploration table transformer used during candidate measurement selection in multi-table relational DP synthesis.

• Strategy Support:
  • Strategy A (empty_token): Extends child attribute domains with an token and emits all P(o, k) permutations with exact weight w = 1 / P(o, k).
  • Strategy B (size_sliced): Operates on native domains and clone tiles children for single-child households.
• Vectorized relational indexing and 2D NumPy grid permutation broadcasting batched by distinct family size k.
• Verification: Comprehensive unit test suite asserting exact marginal symmetry P(S1) == P(S2), total mass conservation (sum(w) == N_parents), and edge cases.

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