feat(ast): add type system declaration nodes

Added AST structures for new type system:
- ConceptDecl: Base type definition
- SubConceptDecl: Enum or record subtype with SubConceptKind
- ConceptComparisonDecl: Compile-time enum mapping
- ConceptMapping: Individual mapping entry

Added Declaration enum variants and stubs throughout codebase
for LSP, resolve, and semantic analysis modules.

src/lsp/hover.rs

@@ -179,6 +179,9 @@ fn get_declaration_name(decl: &Declaration) -> Option<String> {
         | Declaration::Schedule(s) => Some(s.name.clone()),
         | Declaration::Behavior(b) => Some(b.name.clone()),
         | Declaration::Use(_) => None,
+        | Declaration::Concept(_) |
+        Declaration::SubConcept(_) |
+        Declaration::ConceptComparison(_) => None, // TODO: Implement hover for type system
     }
 }
 
@@ -196,6 +199,9 @@ fn format_declaration_hover(decl: &Declaration, _kind: &DeclKind) -> Hover {
         | Declaration::Schedule(s) => format_schedule_hover(s),
         | Declaration::Behavior(b) => format_behavior_hover(b),
         | Declaration::Use(_) => "**use** declaration".to_string(),
+        | Declaration::Concept(_) => "**concept** declaration".to_string(),
+        | Declaration::SubConcept(_) => "**sub_concept** declaration".to_string(),
+        | Declaration::ConceptComparison(_) => "**concept_comparison** declaration".to_string(),
     };
 
     Hover {