improvise-eccl: pool each distinct filtered context once (Codex review on #203)

An inline WHERE on the folded category rewrote every member context to the same item, so SUM(Revenue WHERE Month = Jan) on the Q1 fold counted Jan twice.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/test-evidence/core.md

@@ -1397,3 +1397,15 @@ asserts that there is no Q1 row and a total of 100. That half already held, and
 is the inconsistency the ticket named: before, a hidden value appeared only while
 a sibling was visible. Rival: `GridLayout::evaluate` covering every member
 (`&|_, _| true`). This test alone fails, with 107.
+
+### a_folds_aggregate_filtered_on_the_folded_category_counts_each_record_once
+
+Found by the ChatGPT Codex review of PR #203, at head `14acf46d`. Pooling an
+aggregate over a fold's member contexts ran `agg_values` once per member, and an
+inline `WHERE` on the **folded** category overwrites that coordinate, as it does
+at a record. Every member therefore became the same record set.
+`SUM(Revenue WHERE Month = "Jan")` on the Q1 fold read **20**, Jan twice. The
+test asserts 10, and equality with the Month-on-None document. The fix dedupes
+the member contexts after the filter is applied, so each record set is pooled
+once. The rival is the pre-fix code, which read 20. `a_folds_aggregate_formula_pools_its_members_records`
+could not see this, because none of its aggregates carried a filter.

crates/improvise-core/src/model/types.rs

@@ -62,7 +62,16 @@ fn eval_agg(
     agg_filter: Option<&Filter>,
     eval_at: &dyn Fn(&Expr, &CellKey) -> Result<f64, String>,
 ) -> Result<f64, String> {
-    let values = contexts
+    // A filter overrides its coordinate, so members it rewrites to one item name
+    // one record set: pool each distinct context once (core.md §14).
+    let distinct: IndexSet<CellKey> = contexts
+        .iter()
+        .map(|c| match agg_filter {
+            Some(f) => c.clone().with(&f.category, &f.item),
+            None => c.clone(),
+        })
+        .collect();
+    let values = distinct
         .iter()
         .map(|c| agg_values(model, c, target_category, inner, agg_filter, eval_at))
         .collect::<Result<Vec<_>, String>>()?

crates/improvise-core/src/view/layout.rs

@@ -1829,6 +1829,27 @@ mod tests {
         assert_eq!(grid_value(&mut wb, "Q1", "Top"), Some(30.0));
     }
 
+    /// An inline `WHERE` on the folded category makes every member context the
+    /// same record set — see context/test-evidence/core.md.
+    #[test]
+    fn a_folds_aggregate_filtered_on_the_folded_category_counts_each_record_once() {
+        let formula = ["Jans = SUM(Revenue WHERE Month = \"Jan\")"];
+        let mut folded = margin_book(&formula);
+        folded
+            .active_view_mut()
+            .toggle_group_collapse("Month", "Q1");
+        let fold = grid_value(&mut folded, "Q1", "Jans");
+
+        let mut none = margin_book(&formula);
+        none.active_view_mut().set_axis("Month", Axis::None);
+        assert_eq!(fold, Some(10.0), "Jan's revenue, once");
+        assert_eq!(
+            fold,
+            grid_value(&mut none, "", "Jans"),
+            "the None-axis answer"
+        );
+    }
+
     /// Every row's value in column 0, summed: the totals strip's arithmetic.
     fn column_total(wb: &Workbook) -> f64 {
         let layout = GridLayout::new(&wb.model, wb.active_view());