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.

elb.el

@@ -234,7 +234,9 @@ fn compile_module(src_path: String, out_dir: String, elc_bin: String, dry_run: B
         return true
     }
 
-    let cmd: String = elc_bin + " --emit-header " + src_path + " " + c_out
+    // elc streams C to stdout (collect mode not yet implemented); use
+    // shell redirection so the output lands in the file, not the terminal.
+    let cmd: String = elc_bin + " --emit-header " + src_path + " > " + c_out + " 2>&1"
     println("  compile  " + src_path)
 
     if dry_run { return true }