refactor(specctra-core): Remove `layer_count` from `SpecctraMesadata`

This information is already stored in `Drawing`, I somehow missed that
when I was adding this field.

crates/specctra-core/src/mesadata.rs

@@ -27,9 +27,6 @@ pub trait AccessMesadata: AccessRules + std::panic::RefUnwindSafe {
     /// Retrieves the index of a layer from its name.
     fn layername_layer(&self, layername: &str) -> Option<usize>;
 
-    /// Return the number of the layers.
-    fn layer_count(&self) -> usize;
-
     /// Renames a net based on its index.
     fn bename_net(&mut self, net: usize, netname: String);
 
@@ -79,9 +76,6 @@ pub struct SpecctraMesadata {
     /// These rules are applied to all nets belonging to the respective net clas
     class_rules: BTreeMap<String, SpecctraRule>,
 
-    /// Number of layers.
-    layer_count: usize,
-
     // layername <-> layer for Layout
     /// A bidirectional map between layer indices and layer names, allowing translation
     /// between index-based layers in the layout and user-defined layer names.
@@ -164,7 +158,6 @@ impl SpecctraMesadata {
             structure_rule: SpecctraRule::from_dsn(&structure_rule),
             class_rules,
             layer_layername,
-            layer_count,
             net_netname,
             net_netclass,
         }
@@ -212,10 +205,6 @@ impl AccessMesadata for SpecctraMesadata {
         self.layer_layername.get_by_right(layername).copied()
     }
 
-    fn layer_count(&self) -> usize {
-        self.layer_count
-    }
-
     fn bename_net(&mut self, net: usize, netname: String) {
         self.net_netname.insert(net, netname);
     }

src/autorouter/ratsnests.rs

@@ -12,7 +12,7 @@ pub struct Ratsnests(Box<[Ratsnest]>);
 impl Ratsnests {
     pub fn new(board: &Board<impl AccessMesadata>) -> Result<Self, InsertionError> {
         Ok(Self(
-            (0..board.mesadata().layer_count())
+            (0..board.layout().drawing().layer_count())
                 .map(|principal_layer| Ratsnest::new(board, principal_layer))
                 .collect::<Result<Vec<_>, _>>()
                 .map(Vec::into_boxed_slice)?,