test: add regression test for slice sequence on insert

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/tests/slice-sequence-insert.test.ts

@@ -0,0 +1,51 @@
+/**
+ * Regression test for #3697 — set slice sequence on insert
+ *
+ * All three insertSlice call sites must pass a sequence value so slices
+ * are ordered correctly instead of defaulting to 0.
+ */
+
+import { describe, test } from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+
+const planMilestoneSrc = readFileSync(
+  join(__dirname, '..', 'tools', 'plan-milestone.ts'),
+  'utf-8',
+);
+const reassessRoadmapSrc = readFileSync(
+  join(__dirname, '..', 'tools', 'reassess-roadmap.ts'),
+  'utf-8',
+);
+const mdImporterSrc = readFileSync(
+  join(__dirname, '..', 'md-importer.ts'),
+  'utf-8',
+);
+
+describe('slice sequence on insert (#3697)', () => {
+  test('plan-milestone.ts passes sequence to insertSlice', () => {
+    assert.match(planMilestoneSrc, /insertSlice\(/,
+      'plan-milestone.ts should call insertSlice');
+    assert.match(planMilestoneSrc, /sequence:\s*i\s*\+\s*1/,
+      'plan-milestone.ts should pass sequence: i + 1');
+  });
+
+  test('reassess-roadmap.ts passes sequence to insertSlice', () => {
+    assert.match(reassessRoadmapSrc, /insertSlice\(/,
+      'reassess-roadmap.ts should call insertSlice');
+    assert.match(reassessRoadmapSrc, /sequence:\s*existingCount\s*\+\s*i\s*\+\s*1/,
+      'reassess-roadmap.ts should pass sequence: existingCount + i + 1');
+  });
+
+  test('md-importer.ts passes sequence to insertSlice', () => {
+    assert.match(mdImporterSrc, /insertSlice\(/,
+      'md-importer.ts should call insertSlice');
+    assert.match(mdImporterSrc, /sequence:\s*si\s*\+\s*1/,
+      'md-importer.ts should pass sequence: si + 1');
+  });
+});