docs: record the no-source-text-modules decision; js2c rejects non-.js inputs

Builtins stay classic function bodies on both runtimes; cross-builtin
sharing, if ever needed, starts with generation-time bundling. The js2c
guard turns an accidental .mjs into a build error pointing at the spec.

Author: edusperoni

docs/ns-builtin-modules.md

@@ -123,6 +123,15 @@ provide it.
   the registry distinguishes public modules from internal builtins, and only
   public ones resolve (Node's `canBeRequiredByUsers` split).
 
+## Source-text modules: deliberately not supported
+
+Builtins are classic function bodies, not ES modules, on both runtimes. If
+cross-builtin code sharing is ever needed, the first answer is bundling at
+generation time (author as ESM, emit function bodies); runtime source-text
+builtin modules (Node's `kSourceTextModule`) are justified only by a concrete
+need for live module semantics (TLA, live bindings, cyclic imports), which no
+current or planned builtin has. Revisit here before building either.
+
 ## iOS implementation notes (non-normative)
 
 Builtin modules are function-body builtins (`NativeScript/runtime/js/`,

tools/js2c.mjs

@@ -77,6 +77,11 @@ for (const file of inputs) {
     console.error(`error: no such file: ${file}`);
     process.exit(1);
   }
+  if (!file.endsWith('.js')) {
+    console.error(`error: builtins must be .js function bodies, got: ${file}`);
+    console.error('(.mjs/source-text modules are deliberately unsupported — see docs/ns-builtin-modules.md)');
+    process.exit(1);
+  }
   const stem = basename(file).replace(/\.js$/, '');
   if (!/^[a-z0-9]+(-[a-z0-9]+)*$/.test(stem)) {
     console.error(`error: file name must be kebab-case .js: ${file}`);