docs(context): correct the Eager-math evidence, restore the product-targeted test, record the cycle changes (improvise-bw4p.20)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

crates/improvise-incremental/tests/formulas.rs

@@ -379,3 +379,31 @@ fn ordered_evaluation_keeps_the_zero_eager_settles_on() {
         same_model(&model, &[], &edits, by).unwrap_or_else(|e| panic!("{by:?}: {e}"));
     }
 }
+#[test]
+fn a_new_aggregation_reaches_a_product_targeted_filtered_sum() {
+    let mut model = Model::new("products");
+    for (cat, items) in [("Region", vec!["East", "West"]), ("Product", vec!["F0"])] {
+        model.add_category(cat).unwrap();
+        for item in items {
+            model.category_mut(cat).unwrap().add_item(item);
+        }
+    }
+    model.category_mut("_Measure").unwrap().add_item("Cost");
+    for (region, v) in [("East", 4.0), ("West", 9.0)] {
+        let key = CellKey::new(vec![
+            ("Region".into(), region.into()),
+            ("Product".into(), "F0".into()),
+            ("_Measure".into(), "Cost".into()),
+        ]);
+        model.set_cell(key, CellValue::Number(v));
+    }
+    model.add_formula(parse_formula("P0 = SUM(F0 WHERE Region = East)", "Product").unwrap());
+    let none = vec![];
+    let edits = [Edit::SetMeasureAgg {
+        measure: "Cost".into(),
+        func: improvise_core::formula::ast::AggFunc::Min,
+    }];
+    for by in [Evaluation::Ordered, Evaluation::Plain] {
+        same_model(&model, &none, &edits, by).unwrap();
+    }
+}