Filter pushdown should work with multi-ref filters

[shangyian]

Summary

Builds on the earlier alias/substring fixes. Three correctness changes plus one refactor:

• Multi-reference predicates rewrite fully. Filters like a.x = 1 OR b.y = 2 now rewrite every matching dim ref instead of stopping after the first. If any ref resolves to a column the target CTE doesn't expose, the whole filter stays on the outer query rather than pushing a partial rewrite with an unresolved name behind.
• Set-operation CTE bodies refuse pushdown. Transforms whose body is UNION/INTERSECT/EXCEPT have per-arm projections that can differ, and _inject_filter_into_where only mutates the first arm's WHERE. Pushing would under-filter the other arms.
• Bare column refs in filters now raise. parse_dimension_ref rejects references without a node prefix (e.g., status = 'X') with a clear error directing the user to the fully-qualified form (v3.order_details.status = 'X'). Before, bare refs were silently routed to whichever parent node happened to expose a matching column, which is ambiguous in multi-parent builds.

The rewriter itself moved from regex substitution to a two-pass AST walk that swaps Subscript and Column nodes, then serializes once.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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