fix(#1660): fail-closed frontmatter set of object-list fields instead of silent no-op (#1664)

* fix(#1660): fail-closed frontmatter set of object-list fields instead of silent no-op

cmdFrontmatterSet reported {updated:true} even when spliceFrontmatter returned the
content unchanged, which happened whenever the new value's extractFrontmatter projection
equalled the original's — notably for object-list fields like must_haves, whose
{path,provides} items flatten to scalar strings under the lossy parser. Detect a no-op
(newContent === content) for a dict-valued field and surface an error directing the user
to edit the file directly, instead of silently accepting a no-op set. Scalars and scalar
arrays round-trip faithfully, so idempotent sets of those are intentionally NOT flagged
(two precision regression tests lock this). Folded into frontmatter-cli.test.cjs.

* chore(#1660): backfill changeset pr ref to 1664

* refactor(#1660): extract noOpObjectListSetError as pure tested helper (Stryker coverage)

cmdFrontmatterSet is not in Stryker's property/unit test set, so the inline no-op
detection added survivors that dropped the frontmatter module below its 62% mutation
threshold. Extract the detection into a pure exported helper noOpObjectListSetError and
unit-test every branch directly (changed content, scalar, scalar-array, null, dict
no-op). cmdFrontmatterSet now calls the helper. Same pattern as the #1572 spliceFrontmatter
coverage fix.

tests/frontmatter.unit.test.cjs

@@ -20,6 +20,7 @@ const {
   extractFrontmatter,
   reconstructFrontmatter,
   spliceFrontmatter,
+  noOpObjectListSetError,
   parseMustHavesBlock,
   FRONTMATTER_SCHEMAS,
 } = require('../gsd-core/bin/lib/frontmatter.cjs');
@@ -1202,3 +1203,29 @@ describe('reconstructFrontmatter: nested subval plain string', () => {
     assert.equal(result, 'meta:\n  tag: "issue#42"');
   });
 });
+
+// noOpObjectListSetError (#1660) — pure detection helper, unit-tested directly because the
+// cmdFrontmatterSet path is not in Stryker's property/unit set.
+describe('noOpObjectListSetError (#1660)', () => {
+  const ORIG = '---\nphase: 1\n---\n';
+  test('changed content (real update) → null', () => {
+    assert.equal(noOpObjectListSetError(ORIG, ORIG + 'x', { must_haves: 1 }), null);
+  });
+  test('scalar value no-op → null (idempotent scalar sets are fine)', () => {
+    for (const v of [1, 'str', true, 0, '']) assert.equal(noOpObjectListSetError(ORIG, ORIG, v), null, `scalar ${JSON.stringify(v)}`);
+  });
+  test('scalar-array value no-op → null (scalar arrays round-trip faithfully)', () => {
+    assert.equal(noOpObjectListSetError(ORIG, ORIG, ['a', 'b']), null);
+    assert.equal(noOpObjectListSetError(ORIG, ORIG, []), null);
+  });
+  test('null value no-op → null', () => {
+    assert.equal(noOpObjectListSetError(ORIG, ORIG, null), null);
+  });
+  test('dict value no-op → error message naming the object-list round-trip limit', () => {
+    const msg = noOpObjectListSetError(ORIG, ORIG, { artifacts: [{ path: 'p' }] });
+    assert.equal(typeof msg, 'string');
+    assert.ok(msg.includes('had no effect'), msg);
+    assert.ok(msg.includes('object-list'), msg);
+    assert.ok(msg.includes('Edit the file directly'), msg);
+  });
+});