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-cli.test.cjs

@@ -402,3 +402,42 @@ describe('frontmatter set/merge preserves must_haves object-lists (#1572)', () =
     );
   });
 });
+
+// Bug #1660 — frontmatter set of an object-list field (e.g. must_haves) is a silent no-op
+// when the new value's lossy parse projection equals the original's. Folded into the owning
+// frontmatter-cli test (no new top-level bug-NNNN file).
+describe('Bug #1660: frontmatter set of an object-list field fails closed instead of a silent no-op', () => {
+  const PLAN_WITH_MUST_HAVES = [
+    '---', 'phase: 1', 'wave: 1',
+    'must_haves:', '  artifacts:', '    - path: src/foo.ts', '      provides: the foo',
+    '---', '# body', '',
+  ].join('\n');
+
+  test('setting must_haves to a value that flattens to the original projection fails closed (no silent no-op)', () => {
+    const file = writeTempFile(PLAN_WITH_MUST_HAVES);
+    const before = fs.readFileSync(file, 'utf-8');
+    // New value {artifacts:["path: src/foo.ts"]} — its extractFrontmatter projection equals
+    // the original's flattened projection, so the set would otherwise be a silent no-op.
+    const result = runGsdTools(['frontmatter', 'set', file, '--field', 'must_haves', '--value', JSON.stringify({ artifacts: ['path: src/foo.ts'] })]);
+    const parsed = JSON.parse(result.output);
+    assert.ok(parsed.error, 'a no-op set of an object-list field must surface an error, not silent {updated:true}');
+    const after = fs.readFileSync(file, 'utf-8');
+    assert.equal(after, before, 'the file must be unchanged when the set is refused (no silent partial write)');
+  });
+
+  test('an idempotent set of a scalar (wave, same value) still reports updated (no false positive)', () => {
+    const file = writeTempFile('---\nphase: 1\nwave: 1\n---\n# body\n');
+    const result = runGsdTools(['frontmatter', 'set', file, '--field', 'wave', '--value', '1']);
+    const parsed = JSON.parse(result.output);
+    assert.equal(parsed.updated, true, 'an idempotent SCALAR set must still report {updated:true} (not fail-closed)');
+    assert.ok(!parsed.error, 'an idempotent scalar set must not produce an error');
+  });
+
+  test('an idempotent set of a scalar array (tags, same value) still reports updated (no false positive)', () => {
+    const file = writeTempFile('---\nphase: 1\ntags: ["a","b"]\n---\n# body\n');
+    const result = runGsdTools(['frontmatter', 'set', file, '--field', 'tags', '--value', '["a","b"]']);
+    const parsed = JSON.parse(result.output);
+    assert.equal(parsed.updated, true, 'an idempotent scalar-ARRAY set must still report {updated:true} (arrays round-trip; not fail-closed)');
+    assert.ok(!parsed.error, 'an idempotent scalar-array set must not produce an error');
+  });
+});