fec2af8 introduced temporal bucketing on `PlanNode::ArrangeBy`: the
lowering picks an `ArrangementStrategy` from a `has_future_updates`
analysis bit, and `ensure_collections` inserts a bucketing operator in
front of the arrangement when the strategy is `TemporalBucketing`. Of
the other bucketing-eligible operators, only `Threshold` inherits this
coverage transitively, because its lowering wraps the input in a
synthetic `ArrangeBy` whose target is `new_arranged([required])` (so
`ensure_collections` actually inserts a bucket op there). `TopK` and
`Negate` also wrap their inputs, but the wrap targets `new_raw()` (i.e.,
no actual arrangement), so `ensure_collections` sees
`will_create_arrangement = false` and the wrap's strategy is decorative;
neither operator is bucketed today.

`Reduce` is left out of both hooks: its LIR node has no strategy field,
and its render path (`render_reduce` via `KeyValPlan`) bypasses
`ensure_collections`. As a result, a `Temporal Filter -> GroupAggregate`
pattern — where no `ArrangeBy` sits between the temporal MFP and the
reduce — is never bucketed today.

Add a `temporal_bucketing_strategy: ArrangementStrategy` field to the
`Reduce` LIR node and set it during lowering from the input's
`has_future_updates` flag. At render time, `render_reduce` applies
`apply_bucketing_strategy` to the `(key, val)` stream right before
`render_reduce_plan` arranges it internally.

The lowering also clears `LoweredExpr::has_future_updates` on bucketing
absorption, so a stack of bucketing-eligible operators only buckets at
the lowest one; a trailing temporal MFP fused above naturally re-arms
the flag. The render-side `bucketed: bool` safety net in
`ensure_collections` is still needed to prevent double-bucketing within
a single call (the raw-collection site vs. the `collections.arranged`
loop, or across multiple keys in that loop).

TODO: Union with consolidation, TopK. See next commit.

Follow-up to the Reduce-only temporal-bucketing commit:

- TopK: add `temporal_bucketing_strategy: ArrangementStrategy` to
  `PlanNode::TopK` and `RenderPlan::Expr::TopK`. Lowering populates it
  via `strategy_from_future(input_future)` and clears
  `has_future_updates` when `TemporalBucketing` is chosen. `render_topk`
  calls `apply_bucketing_strategy` on the input stream before building
  the Top-K arrangement(s). Note that this is not ideal for
  `MonotonicTop1Plan { must_consolidate: false }`, where we add temporal
  bucketing with no downstream consolidator; accepted for now and noted
  in a comment. Later, we could move the temporal bucketing decision
  into a new LIR pass.

- Union: add `temporal_bucketing_strategies: Vec<ArrangementStrategy>`
  to `PlanNode::Union` and `RenderPlan::Expr::Union`, lockstep with
  `inputs`. The renderer applies `apply_bucketing_strategy` per leg
  before `concatenate`/`consolidate_named`. Per-leg strategies are
  non-`Direct` only when the Union consolidates its output.

- Threshold: clear `has_future_updates` when the conditional
  `arrange_by` wrap actually fires with `TemporalBucketing`; the
  synthesized `ArrangeBy` (built with `new_arranged`) installs the
  bucket op at render time via `ensure_collections`.

- Fold the `refine_union_negate_consolidation` LIR pass into lowering,
  because we need to know about this when the lowering is deciding about
  temporal bucketing. (Later, we could move the temporal bucketing
  decision into a new LIR pass.)

- Propagate the new fields through EXPLAIN text/json, the
  `PlanNode`/`RenderPlan` interpreter dispatch, and physically-monotonic
  interpretation.

EXPLAIN tests (mirroring commit 623f53d for Reduce):

- src/compute-types/src/explain/text.rs prints
  `temporal_bucketing_strategy=...` for TopK and
  `temporal_bucketing_strategies=[...]` for Union when any strategy is
  non-`Direct`.
- test/sqllogictest/temporal_bucketing.slt: TopK firing (LIMIT over a
  temporal filter), TopK idempotency (over an already-bucketed indexed
  view), Union firing (EXCEPT ALL over two temporal filters).
- test/sqllogictest/explain/physical_plan_as_json.slt: expected JSON
  updated to include the new fields on TopK and Union nodes.

…' can't happen