Move ZoomLevel into skirmish_states

Introduce Structure and Unit traits with enum wrappers.
Replace old zoom helper with ZoomLevel methods.
Update imports, BoardState, CellWidget, and CLI to use new locations.

src/app/states/skirmish_states/structures/stone.rs

@@ -1,20 +1,39 @@
+use crate::app::states::skirmish_states::structures::Structure;
 use ratatui::style::Color;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct Stone {
     durability: u16,
+    stress: u8,
 }
 
 impl Stone {
     pub fn new() -> Self {
-        Self { durability: 1000 }
+        Self {
+            durability: 1000,
+            stress: 0,
+        }
     }
+}
 
-    pub fn get_tag(&self) -> char {
+impl Structure for Stone {
+    fn get_tag(&self) -> char {
         ' '
     }
 
-    pub fn get_color(&self) -> Color {
+    fn get_color(&self) -> Color {
         Color::White
     }
+
+    fn get_level(&self) -> char {
+        ' '
+    }
+
+    fn get_durability(&self) -> u16 {
+        self.durability
+    }
+
+    fn get_stress(&self) -> u8 {
+        self.stress
+    }
 }