Parser: E0505 error on multiple 'start' declarations

Previously a second \`start X\` statement silently overrode the first.
Now emits E0505 at parse time.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/errors/diagnostic.rs

@@ -39,7 +39,6 @@ pub enum ErrorCode {
     E0502, // undefined variable
     E0503, // undefined function
     E0504, // missing start declaration
-    #[allow(dead_code)]
     E0505, // multiple start declarations
 
     // W01xx: Warnings

src/parser/mod.rs

@@ -163,8 +163,16 @@ impl Parser {
                     on_frame = Some(self.parse_on_frame()?);
                 }
                 TokenKind::KwStart => {
+                    let kw_span = self.current_span();
                     self.advance();
                     let (name, _) = self.expect_ident()?;
+                    if start_state.is_some() {
+                        return Err(Diagnostic::error(
+                            ErrorCode::E0505,
+                            "multiple 'start' declarations",
+                            kw_span,
+                        ));
+                    }
                     start_state = Some(name);
                 }
                 _ => {

src/parser/tests.rs

@@ -665,6 +665,22 @@ fn parse_mmc3_mapper() {
     assert_eq!(prog.game.mapper, Mapper::MMC3);
 }
 
+#[test]
+fn parse_multiple_start_declarations_error() {
+    let src = r#"
+        game "Test" { mapper: NROM }
+        state A { on frame { wait_frame } }
+        state B { on frame { wait_frame } }
+        start A
+        start B
+    "#;
+    let errors = parse_err(src);
+    assert!(
+        errors.contains(&crate::errors::ErrorCode::E0505),
+        "duplicate start should produce E0505, got: {errors:?}"
+    );
+}
+
 #[test]
 fn parse_on_scanline_handler() {
     let src = r#"