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/grid.rs

@@ -4,6 +4,115 @@ use crate::view::AxisEntry;
 
 use super::core::{Cmd, CmdContext};
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::command::cmd::test_helpers::*;
+
+    #[test]
+    fn toggle_group_under_cursor_returns_empty_without_groups() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = ToggleGroupAtCursor { is_row: true };
+        let effects = cmd.execute(&ctx);
+        assert!(effects.is_empty());
+    }
+
+    #[test]
+    fn law_toggle_group_involution() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = ToggleGroupAtCursor { is_row: true };
+        let first = effects_debug(&cmd.execute(&ctx));
+        let second = effects_debug(&cmd.execute(&ctx));
+        assert_eq!(first, second, "Toggle should be structurally consistent");
+    }
+
+    #[test]
+    fn view_forward_with_empty_stack_shows_status() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = ViewNavigate { forward: true };
+        let effects = cmd.execute(&ctx);
+        let dbg = effects_debug(&effects);
+        assert!(
+            dbg.contains("No forward view"),
+            "Expected status message, got: {dbg}"
+        );
+    }
+
+    #[test]
+    fn view_back_with_empty_stack_shows_status() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let ctx = make_ctx(&m, &layout, &reg);
+        let cmd = ViewNavigate { forward: false };
+        let effects = cmd.execute(&ctx);
+        let dbg = effects_debug(&effects);
+        assert!(
+            dbg.contains("No previous view"),
+            "Expected status message, got: {dbg}"
+        );
+    }
+
+    #[test]
+    fn view_forward_with_stack_produces_effect() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let fwd_stack = vec!["View 2".to_string()];
+        let mut ctx = make_ctx(&m, &layout, &reg);
+        ctx.view_forward_stack = &fwd_stack;
+        let cmd = ViewNavigate { forward: true };
+        let effects = cmd.execute(&ctx);
+        let dbg = effects_debug(&effects);
+        assert!(
+            dbg.contains("ViewForward"),
+            "Expected ViewForward, got: {dbg}"
+        );
+    }
+
+    #[test]
+    fn view_back_with_stack_produces_apply_and_back() {
+        let m = two_cat_model();
+        let layout = make_layout(&m);
+        let reg = make_registry();
+        let back_stack = vec!["Default".to_string()];
+        let mut ctx = make_ctx(&m, &layout, &reg);
+        ctx.view_back_stack = &back_stack;
+        let cmd = ViewNavigate { forward: false };
+        let effects = cmd.execute(&ctx);
+        assert_eq!(effects.len(), 2);
+        let dbg = effects_debug(&effects);
+        assert!(
+            dbg.contains("ApplyAndClearDrill"),
+            "Expected ApplyAndClearDrill, got: {dbg}"
+        );
+        assert!(dbg.contains("ViewBack"), "Expected ViewBack, got: {dbg}");
+    }
+
+    #[test]
+    fn toggle_prune_empty_produces_toggle_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 = TogglePruneEmpty.execute(&ctx);
+        let dbg = effects_debug(&effects);
+        assert!(
+            dbg.contains("TogglePruneEmpty"),
+            "Expected TogglePruneEmpty, got: {dbg}"
+        );
+    }
+}
+
 // ── Grid operations ─────────────────────────────────────────────────────
 
 /// Toggle the row or column group collapse under the cursor.