fix: always add facet variables to stacking group from spec.facet

Instead of filtering partition_by for facet-prefixed columns, read
facet variables directly from spec.facet.layout.internal_facet_names().
This ensures facet variables are always included in the stacking group
when a facet exists.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cpsievert

src/execute/position.rs

@@ -58,6 +58,7 @@ pub fn apply_position_adjustments(
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::plot::facet::{Facet, FacetLayout};
     use crate::plot::layer::{Geom, Position};
     use crate::plot::{AestheticValue, Mappings, ParameterValue, Scale, ScaleType};
     use polars::prelude::*;
@@ -375,6 +376,9 @@ mod tests {
         let mut spec = Plot::new();
         spec.scales.push(make_discrete_scale("pos1"));
         spec.scales.push(make_continuous_scale("pos2"));
+        spec.facet = Some(Facet::new(FacetLayout::Wrap {
+            variables: vec!["facet_var".to_string()],
+        }));
         let mut data_map = HashMap::new();
         data_map.insert("__ggsql_layer_0__".to_string(), df);
 

src/plot/layer/position/stack.rs

@@ -267,13 +267,10 @@ fn apply_stack(df: DataFrame, layer: &Layer, spec: &Plot, mode: StackMode) -> Re
     // Facet columns must be included so stacking resets per facet panel,
     // matching ggplot2 where position adjustments are computed per-panel.
     let mut over_cols: Vec<Expr> = vec![col(&group_col)];
-    for partition_col in &layer.partition_by {
-        if naming::is_aesthetic_column(partition_col) {
-            if let Some(aes) = naming::extract_aesthetic_name(partition_col) {
-                if aes.starts_with("facet") {
-                    over_cols.push(col(partition_col));
-                }
-            }
+    if let Some(ref facet) = spec.facet {
+        for aes in facet.layout.internal_facet_names() {
+            let facet_col = naming::aesthetic_column(&aes);
+            over_cols.push(col(&facet_col));
         }
     }