feat: add extern let declarations for JS FFI

Add support for `extern let name: Type` and `extern let name: Type = "jsName"`
syntax for declaring external JavaScript values. This follows the same pattern
as extern fn across all compiler passes: parser, typechecker, interpreter
(runtime error placeholder), JS backend (emits JS name directly without
mangling), formatter, linter, modules, and symbol table.

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

src/codegen/js_backend.rs

@@ -73,6 +73,8 @@ pub struct JsBackend {
     used_effects: HashSet<String>,
     /// Extern function names mapped to their JS names
     extern_fns: HashMap<String, String>,
+    /// Extern let names mapped to their JS names
+    extern_lets: HashMap<String, String>,
 }
 
 impl JsBackend {
@@ -96,6 +98,7 @@ impl JsBackend {
             var_substitutions: HashMap::new(),
             used_effects: HashSet::new(),
             extern_fns: HashMap::new(),
+            extern_lets: HashMap::new(),
         }
     }
 
@@ -123,6 +126,13 @@ impl JsBackend {
                     self.extern_fns.insert(ext.name.name.clone(), js_name);
                     self.functions.insert(ext.name.name.clone());
                 }
+                Declaration::ExternLet(ext) => {
+                    let js_name = ext
+                        .js_name
+                        .clone()
+                        .unwrap_or_else(|| ext.name.name.clone());
+                    self.extern_lets.insert(ext.name.name.clone(), js_name);
+                }
                 _ => {}
             }
         }
@@ -1097,6 +1107,9 @@ impl JsBackend {
                 } else if self.functions.contains(&ident.name) {
                     // Function reference (used as value)
                     Ok(self.mangle_name(&ident.name))
+                } else if let Some(js_name) = self.extern_lets.get(&ident.name) {
+                    // Extern let: use JS name directly (no mangling)
+                    Ok(js_name.clone())
                 } else {
                     Ok(self.escape_js_keyword(&ident.name))
                 }