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/resolve/references.rs

@@ -191,6 +191,11 @@ fn find_references_in_declaration(
         | Declaration::Use(_) => {
             // Use statements are handled separately
         },
+        | Declaration::Concept(_) |
+        Declaration::SubConcept(_) |
+        Declaration::ConceptComparison(_) => {
+            // TODO: Implement reference finding for type system
+        },
     }
 
     refs