refactor: colocate cmd tests with their modules

Move tests from the monolithic tests.rs into #[cfg(test)] mod tests
blocks in each command module. Shared test helpers live in
mod.rs::test_helpers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Executed-By: fido

src/command/cmd/cell.rs

@@ -2,6 +2,117 @@ use crate::ui::effect::{self, Effect};
 
 use super::core::{Cmd, CmdContext};
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::command::cmd::test_helpers::*;
+    use crate::model::cell::{CellKey, CellValue};
+
+    #[test]
+    fn clear_selected_cell_produces_clear_and_dirty() {
+        let mut m = two_cat_model();
+        let key = CellKey::new(vec![
+            ("Type".to_string(), "Food".to_string()),
+            ("Month".to_string(), "Jan".to_string()),
+        ]);
+        m.set_cell(key, CellValue::Number(42.0));
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = ClearCellCommand {
+            key: ctx.cell_key().clone().unwrap(),
+        };
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 2);
+    }
+
+    #[test]
+    fn yank_cell_produces_set_yanked() {
+        let mut m = two_cat_model();
+        let key = CellKey::new(vec![
+            ("Type".to_string(), "Food".to_string()),
+            ("Month".to_string(), "Jan".to_string()),
+        ]);
+        m.set_cell(key, CellValue::Number(99.0));
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = YankCell {
+            key: ctx.cell_key().clone().unwrap(),
+        };
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 2);
+    }
+
+    #[test]
+    fn paste_with_yanked_value_produces_set_cell() {
+        let mut m = two_cat_model();
+        m.set_cell(
+            CellKey::new(vec![
+                ("Type".into(), "Food".into()),
+                ("Month".into(), "Jan".into()),
+            ]),
+            CellValue::Number(42.0),
+        );
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let yanked = Some(CellValue::Number(99.0));
+        let mut ctx = make_ctx(&m, &layout, &reg);
+        ctx.yanked = &yanked;
+        let key = CellKey::new(vec![
+            ("Type".into(), "Clothing".into()),
+            ("Month".into(), "Feb".into()),
+        ]);
+        let cmd = PasteCell { key };
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 2);
+        let dbg = effects_debug(&effects);
+        assert!(dbg.contains("SetCell"), "Expected SetCell, got: {dbg}");
+    }
+
+    #[test]
+    fn paste_without_yanked_value_produces_nothing() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let key = CellKey::new(vec![
+            ("Type".into(), "Food".into()),
+            ("Month".into(), "Jan".into()),
+        ]);
+        let cmd = PasteCell { key };
+        let effects = cmd.execute(&ctx);
+        assert!(effects.is_empty());
+    }
+
+    #[test]
+    fn transpose_produces_transpose_and_dirty() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let effects = TransposeAxes.execute(&ctx);
+        assert_eq!(effects.len(), 2);
+        let dbg = effects_debug(&effects);
+        assert!(
+            dbg.contains("TransposeAxes"),
+            "Expected TransposeAxes, got: {dbg}"
+        );
+    }
+
+    #[test]
+    fn save_produces_save_effect() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let effects = SaveCmd.execute(&ctx);
+        assert_eq!(effects.len(), 1);
+        let dbg = effects_debug(&effects);
+        assert!(dbg.contains("Save"), "Expected Save, got: {dbg}");
+    }
+}
+
 // ── Cell operations ──────────────────────────────────────────────────────────
 // All cell commands take an explicit CellKey. The interactive spec fills it
 // from ctx.cell_key(); the parser fills it from Cat/Item coordinate args.