expr: recover pushdown for day-only interval predicates

[antiguru]

Stacked on top of #36702.

Motivation

The parent PR (#36702) fixes the persist filter-pushdown audit panic by marking add/sub_timestamp{,_tz}_interval as non-monotone in both arguments. That's the right correctness answer (calendar-month arithmetic with day-clamping breaks monotonicity for month-bearing intervals), but it's too conservative for the common case of literal day-only intervals — predicates like t_col - INTERVAL '1' day < literal were a motivating use case for filter pushdown and lose their pushdown= line in EXPLAIN.

Description

Extends SpecialBinary with a second handler variant:

enum SpecialBinaryHandler {
    Override(...),            // existing: replace the spec computation entirely
    DynamicMonotone(...),     // new: compute monotonicity flags from the input specs
}

The DynamicMonotone handler for Add/SubTimestamp{,Tz}Interval inspects the right argument's ResultSpec and returns (true, true) if it is a single-value interval with months == 0, else (false, false). The standard flat_map machinery then either narrows precisely or falls back to anything(). Since the static is_monotone annotation isn't consulted when a SpecialBinary is present, the parent PR's (false, false) annotation is still in effect for any code path that bypasses the special handler — so soundness is preserved.

For Trace (which can't see actual values), the SpecialBinary::pushdownable is set to (true, false). This makes t_col +/- INTERVAL_lit Trace-pushdownable regardless of whether the interval has months; at runtime, ColumnSpecs either narrows or hands back anything() based on the dynamic check, so we may attempt pushdown for a month-bearing literal but the predicate will be sound (it just won't narrow anything for that case).

Also restores the pushdown= lines in test/sqllogictest/filter-pushdown.slt that the parent PR had to drop.

Verification

• New regression test interpret::tests::test_timestamp_plus_interval_dynamic_monotone covers three scenarios:
  1. t_col - INTERVAL '1' day < lit over a range that includes both matching and non-matching values → interpreter admits both True and False (tight narrowing).
  2. Same predicate over a range entirely after the literal → interpreter rules out True (impossible matches eliminated).
  3. t_col - INTERVAL '1' month < lit (month-bearing) → interpreter admits both outcomes (conservative fallback; soundness preserved).
• cargo test -p mz-expr --lib — all 52 tests pass.
• test/sqllogictest/filter-pushdown.slt updated to reflect that pushdown= is restored for t - INTERVAL '1' day predicates.

Known limitations

The recovery only kicks in when the interval is a constant with statically-zero months. Day-only column-valued intervals don't currently get pushdown — that's a smaller win to leave for later.

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