Fix escapePDFName producing malformed name escapes for control characters

escapePDFName emitted a single hex digit for character codes below 0x10
(TAB became #9, not #09). PDF 32000-1 7.3.5 requires exactly two hex digits
after #. On re-save (annotations, form fields, font names) such a Name is
written malformed and the lexer mis-parses it on reload, dropping bytes.
Pad the hex to two digits; a no-op for codes 0x10 to 0xFF.

src/core/core_utils.js

@@ -376,7 +376,7 @@ function escapePDFName(str) {
       if (start < i) {
         buffer.push(str.substring(start, i));
       }
-      buffer.push(`#${char.toString(16)}`);
+      buffer.push(`#${char.toString(16).padStart(2, "0")}`);
       start = i + 1;
     }
   }

test/unit/core_utils_spec.js

@@ -290,6 +290,11 @@ describe("core_utils", function () {
         "#23#28#29#3c#3e#5b#5d#7b#7d#2f#25"
       );
     });
+
+    it("should escape control characters using two hexadecimal digits", function () {
+      expect(escapePDFName("\x00\x09\x0a\x1f")).toEqual("#00#09#0a#1f");
+      expect(escapePDFName("a\tb")).toEqual("a#09b");
+    });
   });
 
   describe("escapeString", function () {