fix: emit large integer-valued numbers as <real> to avoid silent corruption

.changeset/large-integer-real.md

@@ -0,0 +1,5 @@
+---
+"plist": patch
+---
+
+Emit large integer-valued numbers (`|x| >= 1e21`) as `<real>` instead of `<integer>`. `Number.prototype.toString()` renders them in exponential notation (`(1e21).toString() === "1e+21"`), which the `<integer>` parse path truncated at the `e`, silently corrupting the value on a `build` → `parse` round-trip.

src/build.ts

@@ -83,7 +83,8 @@ function walk_obj(next: unknown, next_child: xmlbuilder.XMLElement): void {
       }
     }
   } else if (typeof next === 'number') {
-    const tag_type = next % 1 === 0 ? 'integer' : 'real';
+    const tag_type =
+      Number.isInteger(next) && Math.abs(next) < 1e21 ? 'integer' : 'real';
     next_child.ele(tag_type).txt(next.toString());
   } else if (typeof next === 'bigint') {
     next_child.ele('integer').txt(next.toString());

test/parse.test.ts

@@ -40,6 +40,12 @@ describe('parse()', () => {
       const parsed = parseFixture('<integer>14</integer>');
       expect(parsed).toBe(14);
     });
+
+    it('should round-trip integer-valued numbers >= 1e21', () => {
+      expect(parse(build(1e21))).toBe(1e21);
+      expect(parse(build(-1e21))).toBe(-1e21);
+      expect(parse(build(1.23e30))).toBe(1.23e30);
+    });
   });
 
   describe('real', () => {