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/effect_cmds.rs

@@ -4,6 +4,84 @@ use crate::view::Axis;
 
 use super::core::{require_args, Cmd, CmdContext};
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::command::cmd::test_helpers::*;
+
+    #[test]
+    fn add_category_cmd_produces_add_category_effect() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = AddCategoryCmd(vec!["Region".to_string()]);
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 1);
+        let dbg = effects_debug(&effects);
+        assert!(
+            dbg.contains("AddCategory"),
+            "Expected AddCategory, got: {dbg}"
+        );
+    }
+
+    #[test]
+    fn set_cell_cmd_parses_coords_correctly() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = SetCellCmd(vec![
+            "42".to_string(),
+            "Type/Food".to_string(),
+            "Month/Jan".to_string(),
+        ]);
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 1);
+        let dbg = effects_debug(&effects);
+        assert!(dbg.contains("SetCell"), "Expected SetCell, got: {dbg}");
+    }
+
+    #[test]
+    fn set_axis_cmd_recognizes_column_alias() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = SetAxisCmd(vec!["Type".to_string(), "col".to_string()]);
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 1);
+        let dbg = effects_debug(&effects);
+        assert!(dbg.contains("SetAxis"), "Expected SetAxis, got: {dbg}");
+    }
+
+    #[test]
+    fn write_cmd_without_args_saves() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = WriteCmd(vec![]);
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 1);
+        let dbg = effects_debug(&effects);
+        assert!(dbg.contains("Save"), "Expected Save, got: {dbg}");
+    }
+
+    #[test]
+    fn write_cmd_with_path_saves_as() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = WriteCmd(vec!["/tmp/out.improv".to_string()]);
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 1);
+        let dbg = effects_debug(&effects);
+        assert!(dbg.contains("SaveAs"), "Expected SaveAs, got: {dbg}");
+    }
+}
+
 // ── Parseable model-mutation commands ────────────────────────────────────────
 // These are thin Cmd wrappers around effects, constructible from string args.
 // They share the same execution path as keymap-dispatched commands.