Fix O(n²) string construction in codegen-js, lexer, parser, elb

Replace accumulate-by-concatenation loops with native_list_append + str_join.
Eliminates quadratic memory growth when processing large source files.
This is the v2 compiler state — what produced /tmp/elc-v2.

el-compiler/src/parser.el

@@ -687,6 +687,29 @@ fn parse_stmt(tokens: [Map<String, Any>], pos: Int) -> Map<String, Any> {
         return make_result({ "stmt": "Return", "value": val }, p)
     }
 
+    // extern fn declaration (no body — forward declaration for separate compilation)
+    if k == "Extern" {
+        let p = pos + 1
+        let k2: String = tok_kind(tokens, p)
+        if str_eq(k2, "Fn") {
+            let p = p + 1
+            let name: String = tok_value(tokens, p)
+            let p = p + 1
+            let r = parse_params(tokens, p)
+            let params = r["params"]
+            let p = r["pos"]
+            let ret_type = ""
+            let k3: String = tok_kind(tokens, p)
+            if str_eq(k3, "Arrow") {
+                let p = p + 1
+                let kt: String = tok_kind(tokens, p)
+                if str_eq(kt, "Ident") { let ret_type = tok_value(tokens, p) }
+                let p = skip_type(tokens, p)
+            }
+            return make_result({ "stmt": "ExternFn", "name": name, "params": params, "ret_type": ret_type }, p)
+        }
+    }
+
     // fn definition
     if k == "Fn" {
         let p = pos + 1