fix(build): colocateLlmlinguaOptionals skip-check treated a Next-traced stub as fully copied

Debugging the omniroute-beta Docker rebuild: `npm run build` (and the
Dockerfile's own post-build verification) failed with
`Cannot find module '.../node_modules/@atjsh/llmlingua-2/dist/index.js'`.

Root cause, reproduced directly (both against a live Docker builder image
and in a unit test): Next.js's own standalone trace creates a stub
directory for `@atjsh/llmlingua-2` containing only `package.json` — it
references the package (a dynamically-imported optional dependency) but
can't fully bundle it. colocateLlmlinguaOptionals's skip checks (both the
closure-level early return and the per-package loop) only tested
`existsSync(dest)`, so that stub was indistinguishable from "already fully
co-located" — the function skipped copying the real `dist/` output
entirely, silently shipping a package with a manifest but no code.

Fix: check for the package's declared `main` entry file when it has one
(the real-world case for every actual SLM optional). Packages with no
`main` field fall back to comparing the destination's top-level entries
against the source's — correct both for genuinely multi-file packages and
for a metadata-only source (package.json is then its complete, faithfully-
copied contents), which the existing idempotency test exercises.

Covered by tests/unit/colocate-optionals.test.ts's new stub-reproduction
case (fails against the pre-fix code, passes after — confirmed directly)
plus the 6 pre-existing cases, all still green.

tests/unit/colocate-optionals.test.ts

@@ -175,6 +175,41 @@ test("colocateLlmlinguaOptionals skips when there is no standalone dist bundle",
   }
 });
 
+test("colocateLlmlinguaOptionals fills a Next-traced stub (package.json only, no dist) instead of skipping it", () => {
+  // Reproduces a real build failure: Next.js's own standalone trace can create
+  // a stub directory for a dynamically-imported optional dependency it
+  // references but can't fully bundle — just package.json, no actual code.
+  // The old skip check (`existsSync(dest)`) treated that stub as "already
+  // co-located" and never copied the real dist/ output, so
+  // require.resolve('@atjsh/llmlingua-2') found a package.json with no
+  // matching main file at runtime.
+  const root = mkdtempSync(join(tmpdir(), "omniroute-colocate-stub-"));
+  try {
+    buildRoot(root);
+    const distNm = join(root, "dist", "node_modules");
+    mkPkg(distNm, "@huggingface/transformers", { version: "3.5.2" });
+
+    // Simulate the Next-traced stub: directory exists, package.json only.
+    const stubDir = join(distNm, "@atjsh", "llmlingua-2");
+    mkdirSync(stubDir, { recursive: true });
+    writeFileSync(
+      join(stubDir, "package.json"),
+      readFileSync(join(root, "node_modules", "@atjsh", "llmlingua-2", "package.json"), "utf8")
+    );
+    assert.ok(!existsSync(join(stubDir, "dist", "index.js")), "stub must start without dist/");
+
+    const result = colocateLlmlinguaOptionals({ rootDir: root });
+    assert.equal(result.skipped, false, "must not treat the stub as already co-located");
+
+    assert.ok(
+      existsSync(join(stubDir, "dist", "index.js")),
+      "the real dist/index.js must be filled in, not left missing behind the stub"
+    );
+  } finally {
+    rmSync(root, { recursive: true, force: true });
+  }
+});
+
 test("SEED_PACKAGES excludes transformers (it is a dist-pinned peer, not a seed)", () => {
   assert.ok(!SEED_PACKAGES.includes("@huggingface/transformers"));
   assert.deepEqual(SEED_PACKAGES, ["@atjsh/llmlingua-2", "@tensorflow/tfjs", "js-tiktoken"]);