fix(executor): keep the evaluation timestamp on temporal pane merges

A per-series SummaryMerge grouped away the time column, so its native
output disagreed with the Planner schema ("native output type differs
from Planner output"). Merge by series identity, then attach the run
scope's timestamp with scope_timestamp, as per-series SummaryAgg does.
Pane build timestamps never reach the merged state. Ported from #569.

Regression tests merge 5 one-minute panes for an exact Sum and a KLL
quantile, rebuilt at query time and retained with distinct build
timestamps, and compare them with one 5-minute build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: zzylol

crates/executor/src/physical_planner/mod.rs

@@ -551,6 +551,22 @@ fn compile_internal(
                     continue;
                 }
             }
+            if matches!(node.payload, Payload::SummaryMerge) && output.time_index.is_some() {
+                // Pane timestamps describe their individual builds. A merged
+                // per-series state represents this evaluation's entire window,
+                // so merge by series identity and attach the execution scope's
+                // timestamp after merging, as per-series SummaryAgg does.
+                let merged = bind_operation(node, &schemas)?;
+                let compact = merged.schema();
+                let merge_id = helper_id(id, 1);
+                physical_dag.add(merge_id, inputs, merged)?;
+                physical_dag.add(
+                    id,
+                    vec![merge_id],
+                    Operator::scope_timestamp(compact, output)?,
+                )?;
+                continue;
+            }
             let mut operator = compile_node(node, &schemas)
                 .map_err(|error| invalid(format!("node {id}: {error}")))?;
             if operator.is_counter_evaluation() {