feat(model): introduce virtual '_Measure' category for formulas

Refactor formula handling to use a virtual '_Measure' category.

- Formulas now default to targeting '_Measure' if no category is specified.
- '_Measure' items are dynamically computed from existing data items and
  formula targets, preventing redundant item storage.
- Updated persistence to handle '_Measure' formulas and items correctly in
  Markdown.
- Updated UI and model to use 'effective_item_names' for layout and item
  resolution.
- Updated tests to reflect the new '_Measure' behavior.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

src/view/layout.rs

@@ -93,15 +93,7 @@ impl GridLayout {
         let page_coords = page_cats
             .iter()
             .map(|cat| {
-                let items: Vec<String> = model
-                    .category(cat)
-                    .map(|c| {
-                        c.ordered_item_names()
-                            .into_iter()
-                            .map(String::from)
-                            .collect()
-                    })
-                    .unwrap_or_default();
+                let items: Vec<String> = model.effective_item_names(cat);
                 let sel = view
                     .page_selection(cat)
                     .map(String::from)
@@ -518,11 +510,13 @@ fn expand_category(
     let mut result = Vec::new();
     let mut last_group: Option<&str> = None;
 
-    for item_name in cat.ordered_item_names() {
+    // Use effective_item_names so _Measure includes formula targets dynamically
+    let effective_names = model.effective_item_names(cat_name);
+    for item_name in &effective_names {
         if view.is_hidden(cat_name, item_name) {
             continue;
         }
-        let item_group = cat.items.get(item_name).and_then(|i| i.group.as_deref());
+        let item_group = cat.items.get(item_name.as_str()).and_then(|i| i.group.as_deref());
 
         // Emit a group header at each group boundary.
         if item_group != last_group {