Push expressions through layout plans - #9166

Summary

• replace the virtual Plan::optimize_expression hook with static typed parent-child rules
• register rules for (ExpressionPlan, StructPlan), (ExpressionPlan, DictPlan), (ExpressionPlan, ChunkedPlan), and (ExpressionPlan, RowIdxPlan)
• push independent struct-field expressions into each field while retaining cross-field work as a residual expression
• push safe boolean expressions into dictionary values while retaining dictionary codes
• optimize heterogeneous chunks independently while preserving global row-index semantics
• partition mixed row-index/data expressions and combine their outputs with a residual expression
• add complete before/after snapshots, including multi-field struct and row-index/struct composition

Stack

This draft is stacked on #9196 and targets vortex-plan-rules.

Example

Before optimization:

ExpressionPlan((#row_idx > 11) and (($.a > 5) and ($.b > 7)))
  RowIdxPlan
    StructPlan
      a: DictPlan
      b: FlatPlan

After optimization, the row-index predicate is evaluated by RowIdxValuesPlan, the field predicates are pushed into the corresponding struct children, and a residual expression combines the partition outputs. No synthetic struct-partition plan node is introduced.

Safety

• dictionary pushdown requires boolean output, a root reference, strictness, and infallibility
• nullable struct expressions remain above parent validity
• chunk pushdown rejects expressions referencing global row indices
• parent-reduction rules verify row-count and dtype preservation in debug builds
• cross-field expressions that cannot be partitioned remain above the original struct plan

Checks

• cargo test -p vortex-layout — 209 passed
• cargo clippy -p vortex-layout --all-targets --all-features -- -D warnings
• cargo +nightly fmt --all -- --check
• git diff --check