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/syntax/ast.rs

@@ -70,6 +70,9 @@ pub enum Declaration {
     Location(Location),
     Species(Species),
     Enum(EnumDecl),
+    Concept(ConceptDecl),
+    SubConcept(SubConceptDecl),
+    ConceptComparison(ConceptComparisonDecl),
 }
 
 /// Use statement for importing definitions
@@ -414,6 +417,46 @@ pub struct EnumDecl {
     pub span: Span,
 }
 
+/// Concept declaration - base type definition
+#[derive(Debug, Clone, PartialEq)]
+pub struct ConceptDecl {
+    pub name: String,
+    pub span: Span,
+}
+
+/// Sub-concept declaration - enum or record subtype
+#[derive(Debug, Clone, PartialEq)]
+pub struct SubConceptDecl {
+    pub name: String,
+    pub parent_concept: String,
+    pub kind: SubConceptKind,
+    pub span: Span,
+}
+
+/// Sub-concept can be either enum-like or record-like
+#[derive(Debug, Clone, PartialEq)]
+pub enum SubConceptKind {
+    Enum { variants: Vec<String> },
+    Record { fields: Vec<Field> },
+}
+
+/// Concept comparison - compile-time enum mapping between two concepts
+#[derive(Debug, Clone, PartialEq)]
+pub struct ConceptComparisonDecl {
+    pub name: String,
+    pub left_concept: String,
+    pub right_concept: String,
+    pub mappings: Vec<ConceptMapping>,
+    pub span: Span,
+}
+
+/// A single mapping entry in a concept comparison
+#[derive(Debug, Clone, PartialEq)]
+pub struct ConceptMapping {
+    pub left_variant: String,
+    pub right_variant: String,
+}
+
 /// Expression AST for conditions and queries
 #[derive(Debug, Clone, PartialEq)]
 pub enum Expr {