fix: percent-encode X-File-Results header to support non-ASCII filenames

The X-File-Results header contained raw JSON with non-ASCII characters
from filenames (Chinese, Japanese, etc.), violating RFC 7230. Node.js
threw ERR_INVALID_CHAR on writeHead(). Fixed by wrapping the JSON in
encodeURIComponent() on the backend and decodeURIComponent() on the
frontend, ensuring only ASCII goes into the header while preserving
the original filenames after decoding.

Closes #133

tests/integration/adversarial-extended.test.ts

@@ -649,7 +649,7 @@ describe("Batch edge cases — extended", () => {
     // Should succeed and deduplicate filenames in the ZIP
     expect(res.statusCode).toBe(200);
     expect(res.headers["content-type"]).toBe("application/zip");
-    const fileResults = JSON.parse(res.headers["x-file-results"] as string);
+    const fileResults = JSON.parse(decodeURIComponent(res.headers["x-file-results"] as string));
     // All three entries should have unique names
     const names = Object.values(fileResults);
     expect(new Set(names).size).toBe(3);
@@ -1524,7 +1524,7 @@ describe("Batch limits — boundary tests", () => {
 
     expect(res.statusCode).toBe(200);
     expect(res.headers["content-type"]).toBe("application/zip");
-    const fileResults = JSON.parse(res.headers["x-file-results"] as string);
+    const fileResults = JSON.parse(decodeURIComponent(res.headers["x-file-results"] as string));
     expect(Object.keys(fileResults).length).toBe(1);
   });
 
@@ -1546,7 +1546,7 @@ describe("Batch limits — boundary tests", () => {
 
     expect(res.statusCode).toBe(200);
     expect(res.headers["content-type"]).toBe("application/zip");
-    const fileResults = JSON.parse(res.headers["x-file-results"] as string);
+    const fileResults = JSON.parse(decodeURIComponent(res.headers["x-file-results"] as string));
     expect(Object.keys(fileResults).length).toBe(10);
   }, 120_000);