test(train-stamp): S_i keeps the base's four-part package.json (#3557)

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -22,7 +22,7 @@
 - `test/ship-queue-land.test.ts` (bare origin and a gh shim): happy path; two PRs from one base without a collision; a legacy stamp plus a dependency edit; a title-derived entry; a product conflict; the lock and `--break-lock`; the base moving during CI; a foreign push never overwritten; a missing receipt; drift, `--repair` and the lock release; the census; a receipt that fails #3567's depth check refuses; a base with a four-part package.json in `stamp_files` lands four parts (#3557), gstack's own shape stays three.
 - `test/version-gate-fragments.test.ts`: fragment heads, premature version changes, a bot author, an advanced base, an invalid fragment, the stamp commit's effective bump, and the workflow's missing path filter.
 - `test/restamp-command-boundary.test.ts`: the hook reads its counter from `GSTACK_RESTAMP_BASE_SHA`, not the branch.
-- `test/ship-queue-train-stamp.test.ts`: S2 keeps S1's release and runs the versions in order with each slot's bump; a stacked child folds only its own fragment; a tampered earlier version or a product change is refused.
+- `test/ship-queue-train-stamp.test.ts`: S2 keeps S1's release and runs the versions in order with each slot's bump; a stacked child folds only its own fragment; a four-part package.json base keeps four parts in every S_i, read from S_(i-1); a tampered earlier version or a product change is refused.
 - `test/ship-queue-train-land.test.ts`: build records S1 and S2 and gates S2; land publishes each slot exactly with the label; a foreign push is never overwritten; a merge outside the train freezes the queue; a plain train refuses `train land`.
 - `test/train-backstop.test.ts`: main equal to S1 is covered; the same tree with another VERSION is gated; a chain mixing stamped and plain prefixes is refused.
 

test/ship-queue-train-stamp.test.ts

@@ -67,6 +67,21 @@ describe('stamped train prefixes', () => {
     expect(top).not.toContain('- parent change');
   }, 60_000);
 
+  test('a base with a four-part package.json listed in stamp_files keeps four parts in every S_i, read from S_(i-1)', () => {
+    const policy = { ...POLICY, stamp_files: ['VERSION', 'package.json', 'CHANGELOG.md', 'changelog.d/*.md'], stamp_patterns: ['"version": "{version}"'] };
+    const r = makeFixtureRepo({ policy, prefix: 'train-stamp-' }); repos.push(r);
+    const pkg = JSON.parse(fs.readFileSync(path.join(r.work, 'package.json'), 'utf8'));
+    fs.writeFileSync(path.join(r.work, 'package.json'), JSON.stringify({ ...pkg, version: '1.2.3.0' }, null, 2) + '\n');
+    fs.rmSync(path.join(r.work, 'package-lock.json'));
+    r.git('add', '-A'); r.git('commit', '-q', '-m', 'four-part manifest'); r.git('push', '-q', 'origin', 'main');
+    const a = pin(r, 'a', 'origin/main', '- a change\n');
+    const b = pin(r, 'b', 'origin/main', '- b change\n');
+    const o = { ...chain(r, [['1', a], ['2', b]]), policy: mergePolicy(policy, 'test').policy };
+    const out = stampChain(o);
+    if ('code' in out) throw new Error(out.message);
+    expect(out.stamped.map(s => JSON.parse(show(r, s.commit + ':package.json')).version)).toEqual(['1.2.4.0', '1.2.5.0']);
+  }, 60_000);
+
   test('a tampered earlier version or a product change in S_i is refused', () => {
     const r = makeFixtureRepo({ policy: POLICY, prefix: 'train-stamp-' }); repos.push(r);
     const a = pin(r, 'a', 'origin/main', '- a change\n');