fix(votes): sort tasks before filtering to preserve stable order

Previously sort() was applied after filter(), which mutated a derived array
on every keystroke. Pre-sort via sortedTasks derived state ensures stable
order regardless of search input.

Also strengthen test: use typical90 fixture to isolate contest name label
matching, and add order-preservation test.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: KATO-Hiro

src/routes/votes/+page.svelte

@@ -29,9 +29,8 @@
 
   let search = $state('');
 
-  const filteredTasks = $derived(
-    filterTasksBySearch(data.tasks, search, MAX_SEARCH_RESULTS).sort(compareByContestIdAndTaskId),
-  );
+  const sortedTasks = $derived([...data.tasks].sort(compareByContestIdAndTaskId));
+  const filteredTasks = $derived(filterTasksBySearch(sortedTasks, search, MAX_SEARCH_RESULTS));
 </script>
 
 <div class="container mx-auto w-5/6">

src/test/lib/utils/task_filter.test.ts

@@ -7,6 +7,8 @@ const tasks = [
   { title: 'AGC 060 C - No Majority', task_id: 'agc060_c', contest_id: 'agc060' },
   { title: 'ABC 301 A - Overall Winner', task_id: 'abc301_a', contest_id: 'abc301' },
   { title: 'ABC 302 A - Attack', task_id: 'abc302_a', contest_id: 'abc302' },
+  // title intentionally omits '典型' so only getContestNameLabel matches '競プロ典型 90 問'
+  { title: 'Shortest Path', task_id: 'typical90_a', contest_id: 'typical90' },
 ];
 
 describe('filterTasksBySearch', () => {
@@ -32,11 +34,11 @@ describe('filterTasksBySearch', () => {
     expect(result[0].task_id).toBe('agc060_c');
   });
 
-  test('matches by contest name label (e.g. "ABC 300")', () => {
-    // getContestNameLabel('abc300') => 'ABC 300'
-    const result = filterTasksBySearch(tasks, 'ABC 300', 20);
+  test('matches by contest name label when title does not contain the label', () => {
+    // getContestNameLabel('typical90') => '競プロ典型 90 問'; title is 'Shortest Path' — no overlap
+    const result = filterTasksBySearch(tasks, '競プロ典型 90 問', 20);
     expect(result).toHaveLength(1);
-    expect(result[0].task_id).toBe('abc300_a');
+    expect(result[0].task_id).toBe('typical90_a');
   });
 
   test('returns empty array when no tasks match', () => {
@@ -49,6 +51,14 @@ describe('filterTasksBySearch', () => {
     expect(result).toHaveLength(2);
   });
 
+  test('preserves input order — sorting is the caller responsibility', () => {
+    // tasks are declared as: abc300, arc150, agc060, abc301, abc302
+    // abc30x matches: abc300, abc301, abc302 (in that input order)
+    // With limit=3, the first 2 from input order should be returned when limit=2
+    const result = filterTasksBySearch(tasks, 'abc30', 3);
+    expect(result.map((t) => t.task_id)).toEqual(['abc300_a', 'abc301_a', 'abc302_a']);
+  });
+
   test('returns all matched results when count is less than limit', () => {
     const result = filterTasksBySearch(tasks, 'arc', 20);
     expect(result).toHaveLength(1);