Language: enum declarations

Adds \`enum Name { V1, V2, ... }\` as a top-level declaration. Each
variant is registered as a \`u8\` constant equal to its index in the
declaration. Variant names are global and must be unique across all
enums and other symbols (E0501 on collision).

- Lexer: \`enum\` keyword
- AST: \`EnumDecl { name, variants, span }\` field on \`Program\`
- Parser: top-level \`enum\` syntax with optional trailing commas
- Analyzer: \`register_enum\` flattens variants into the symbol table
- IR lowering and AST codegen: variants resolve through the existing
  \`const_values\` path
- Tests cover parsing, duplicate detection, and usage

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/analyzer/tests.rs

@@ -298,6 +298,42 @@ fn analyze_return_wrong_type() {
     );
 }
 
+#[test]
+fn analyze_enum_variants_as_constants() {
+    let result = analyze_ok(
+        r#"
+        game "Test" { mapper: NROM }
+        enum Color { Red, Green, Blue }
+        var c: u8 = Red
+        on frame {
+            if c == Blue { c = Green }
+        }
+        start Main
+    "#,
+    );
+    // Variants should be registered as constant symbols.
+    assert!(result.symbols.get("Red").is_some_and(|s| s.is_const));
+    assert!(result.symbols.get("Green").is_some_and(|s| s.is_const));
+    assert!(result.symbols.get("Blue").is_some_and(|s| s.is_const));
+}
+
+#[test]
+fn analyze_duplicate_enum_variant_errors() {
+    let errors = analyze_errors(
+        r#"
+        game "Test" { mapper: NROM }
+        enum A { Foo, Bar }
+        enum B { Baz, Bar }
+        on frame { wait_frame }
+        start Main
+    "#,
+    );
+    assert!(
+        errors.contains(&ErrorCode::E0501),
+        "duplicate variant should emit E0501, got: {errors:?}"
+    );
+}
+
 #[test]
 fn analyze_dead_code_after_break() {
     let src = r#"