fix: allocate export names in linear time

src/onenote/export/export.test.ts

@@ -436,6 +436,28 @@ describe('OneNote export filenames', () => {
     expect(allocator.allocate('Report.txt')).toBe('Report.txt');
     expect(allocator.allocate('report.TXT')).toBe('report-2.txt');
   });
+
+  it('keeps collision suffixes within the filename cap', () => {
+    const allocator = new SafeExportNameAllocator();
+    const filename = `${'x'.repeat(116)}.txt`;
+    expect(filename).toHaveLength(120);
+    expect(allocator.allocate(filename)).toBe(filename);
+
+    const duplicate = allocator.allocate(filename);
+    expect(duplicate).toBe(`${'x'.repeat(114)}-2.txt`);
+    expect([...duplicate]).toHaveLength(120);
+  });
+
+  it('allocates many duplicate names without rescanning prior suffixes', () => {
+    const allocator = new SafeExportNameAllocator();
+    const names = Array.from({ length: 10_000 }, () =>
+      allocator.allocate('Report.txt')
+    );
+
+    expect(names[0]).toBe('Report.txt');
+    expect(names.at(-1)).toBe('Report-10000.txt');
+    expect(new Set(names)).toHaveLength(names.length);
+  });
 });
 
 function fixtureSnapshot(): OneNoteExportSnapshot {