Analyzer: E0502 on assignment to undefined variable

Previously an assignment to an undeclared name was silently accepted
because the IR lowering would synthesize a fresh VarId for it. Now
the analyzer emits E0502 with the same "did you mean" suggestion
path as read-side undefined variable errors.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/analyzer/mod.rs

@@ -797,6 +797,11 @@ impl Analyzer {
                                     *span,
                                 ));
                             }
+                        } else {
+                            // Assigning to an undeclared name is an
+                            // error — the lowering would otherwise
+                            // silently synthesize a VarId for it.
+                            self.emit_undefined_var(name, *span);
                         }
                     }
                     LValue::ArrayIndex(name, idx) => {
@@ -808,6 +813,8 @@ impl Analyzer {
                                     *span,
                                 ));
                             }
+                        } else {
+                            self.emit_undefined_var(name, *span);
                         }
                         // Indexing an array counts as a read of the array,
                         // and the index expression itself may contain reads.

src/analyzer/tests.rs

@@ -360,6 +360,23 @@ fn analyze_unknown_struct_type_errors() {
     );
 }
 
+#[test]
+fn analyze_assign_to_undefined_var_errors() {
+    // Assigning to an undeclared variable must produce E0502
+    // rather than silently creating the variable.
+    let errors = analyze_errors(
+        r#"
+        game "Test" { mapper: NROM }
+        on frame { nope = 5 }
+        start Main
+    "#,
+    );
+    assert!(
+        errors.contains(&ErrorCode::E0502),
+        "assignment to undefined var should produce E0502, got: {errors:?}"
+    );
+}
+
 #[test]
 fn analyze_enum_variants_as_constants() {
     let result = analyze_ok(