feat(optimize): push filter through Distinct nodes

[protosphinx]

Why

Predicate pushdown skips Distinct nodes today, leaving a Filter
sitting above a Distinct after optimization. This is suboptimal: since
predicates are pure, filter(distinct(X), pred) = distinct(filter(X, pred)).
Moving the filter below the dedup step shrinks the row set before the
Distinct operator sees it, reducing both the cardinality it must process
and the overall cost.

The motivating case is WITH DISTINCT ... WHERE ...: the WHERE lands
above the Distinct in the plan, and previously the optimizer could not
push it further.

What

• src/optimize.rs: add a Plan::Distinct arm to try_push_filter
  that recurses through the Distinct node, pushing the filter into its
  input (6 lines).
• tests/distinct_optimize.rs: 7 new integration tests covering
  passthrough-var push, renamed-alias push, plan-shape position check,
  optimizer idempotence for RETURN DISTINCT and WITH DISTINCT + WHERE,
  cost improvement, and descend-into-RETURN-DISTINCT-input.

Tests

• cargo test - 213 tests pass (206 existing + 7 new)
• cargo clippy --all-targets -- -D warnings - clean
• cargo fmt --check - clean

Self-merge gate

• all CI checks pass
• LOC delta < 250 (152 lines: 6 in src/optimize.rs, 146 in tests/)
• no public-API surface change (src/lib.rs not modified)
• no runtime-dependency additions
• no workflow file changes
• tests added (tests/distinct_optimize.rs, 7 tests)

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Generated by Claude Code