Enumerate finite intersections from a finite child; gather seeds from all children

[siddhss5]

Why

#55: members(AllOf) delegated to seeds(), which selected the first child with any explicit seed. A child can carry seeds without being finite, so an intersection whose first seed-bearing child was a mixed union enumerated nothing even though a later child was finite. Reproduced on main: the issue's example is finite, contains [1, 0], and returned empty members and seeds.

What

• members has its own recursion again. A union enumerates all children when every child is finite. An intersection enumerates a child that is itself finite (the first such) and keeps the members every other child contains; the configurations returned do not depend on child order, only their provenance does.
• seeds(AllOf) returns the exhaustive members when the intersection is finite. Otherwise it collects seeds from every seed-bearing child, keeps those the whole intersection contains, and deduplicates equal configurations by first occurrence in child order. Documented as the policy.
• Mixed finite/sampleable union behavior from Preserve explicit roots in mixed finite and sampleable unions #42 is unchanged.

Fixes #55. Refs #17, #42.

Test plan

Six new tests in test_seeds.py:

• the issue's reproduction enumerates [1, 0] and it becomes a tree root;

• members configurations are identical under three child orderings of an intersection;

• a non-finite intersection whose seed-bearing children have disjoint finite parts yields no seeds, and one with a shared configuration yields it once with the earliest provenance (first child bears no seeds);

• the first-occurrence dedup policy;

• hypothesis over random nested AnyOf/AllOf trees of finite sets, predicates, and samplers: whenever is_finite(s), the set of members(s) configurations equals the membership filter of all leaf configurations, seeds(s) agrees, and otherwise members is empty and every seed is a member.

• uv run pytest tests/: 261 passed, 1 skipped; ruff clean.

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