feat: add Ref cells for mutable state (Ref.new, Ref.get, Ref.set, Ref.update)

Implements WISH-013 mutable state primitives. Ref<T> is a mutable container
using existing module call syntax. Supported across interpreter, JS, and C backends.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/codegen/js_backend.rs

@@ -1242,6 +1242,9 @@ impl JsBackend {
                         if module_name.name == "Map" {
                             return self.emit_map_operation(&field.name, args);
                         }
+                        if module_name.name == "Ref" {
+                            return self.emit_ref_operation(&field.name, args);
+                        }
                     }
                 }
 
@@ -1388,6 +1391,11 @@ impl JsBackend {
                     return self.emit_map_operation(&operation.name, args);
                 }
 
+                // Special case: Ref module operations (not an effect)
+                if effect.name == "Ref" {
+                    return self.emit_ref_operation(&operation.name, args);
+                }
+
                 // Special case: Html module operations (not an effect)
                 if effect.name == "Html" {
                     return self.emit_html_operation(&operation.name, args);
@@ -2486,6 +2494,37 @@ impl JsBackend {
         }
     }
 
+    fn emit_ref_operation(
+        &mut self,
+        operation: &str,
+        args: &[Expr],
+    ) -> Result<String, JsGenError> {
+        match operation {
+            "new" => {
+                let val = self.emit_expr(&args[0])?;
+                Ok(format!("({{value: {}}})", val))
+            }
+            "get" => {
+                let r = self.emit_expr(&args[0])?;
+                Ok(format!("({}.value)", r))
+            }
+            "set" => {
+                let r = self.emit_expr(&args[0])?;
+                let val = self.emit_expr(&args[1])?;
+                Ok(format!("({}.value = {}, undefined)", r, val))
+            }
+            "update" => {
+                let r = self.emit_expr(&args[0])?;
+                let f = self.emit_expr(&args[1])?;
+                Ok(format!("({0}.value = {1}({0}.value), undefined)", r, f))
+            }
+            _ => Err(JsGenError {
+                message: format!("Unknown Ref operation: {}", operation),
+                span: None,
+            }),
+        }
+    }
+
     /// Emit Html module operations for type-safe HTML construction
     fn emit_html_operation(
         &mut self,