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.

src/frontmatter.cts

@@ -503,10 +503,35 @@ function cmdFrontmatterSet(cwd: string, filePath: string, field: string | undefi
   try { parsedValue = JSON.parse(value as string); } catch { parsedValue = value; }
   fm[field as string] = parsedValue as FrontmatterValue;
   const newContent = spliceFrontmatter(content, fm);
+  // #1660: a no-op set (newContent unchanged) with a dict-valued field means the lossy
+  // frontmatter parser made the new value's projection equal the original's — the change
+  // did not apply (bites object-list fields like must_haves). Detection lives in the pure
+  // exported helper noOpObjectListSetError so the mutation gate (property/unit set) covers
+  // it — the cmd path itself is not in that set.
+  const noOpErr = noOpObjectListSetError(content, newContent, parsedValue);
+  if (noOpErr) {
+    output({ error: noOpErr, field }, raw, undefined);
+    return;
+  }
   platformWriteSync(fullPath, newContent);
   output({ updated: true, field, value: parsedValue }, raw, 'true');
 }
 
+/**
+ * #1660: detect a frontmatter `set` that would be a silent no-op on a dict-valued field.
+ * Returns an error message when the splice produced no content change but the new value
+ * is a dict (object-list fields like must_haves, whose `{path, provides}` items flatten to
+ * scalar strings under extractFrontmatter so a replacement can deep-equal the original's
+ * projection), else null. Scalars and scalar arrays round-trip faithfully, so idempotent
+ * sets of those are intentionally NOT flagged. Pure and unit-tested directly (the cmd path
+ * is not in Stryker's property/unit set, so the detection must be testable in isolation).
+ */
+function noOpObjectListSetError(originalContent: string, newContent: string, parsedValue: unknown): string | null {
+  if (newContent !== originalContent) return null;
+  if (parsedValue === null || typeof parsedValue !== 'object' || Array.isArray(parsedValue)) return null;
+  return 'frontmatter set had no effect — the supplied value is equivalent to the existing field under the frontmatter parser, which cannot faithfully round-trip object-list fields like must_haves. Edit the file directly.';
+}
+
 function cmdFrontmatterMerge(cwd: string, filePath: string, data: string | undefined, raw: boolean): void {
   if (!filePath || !data) { error('file and data required'); }
   const fullPath = path.isAbsolute(filePath) ? filePath : path.join(cwd, filePath);
@@ -543,6 +568,7 @@ export = {
   parseFrontmatter: extractFrontmatter,
   reconstructFrontmatter,
   spliceFrontmatter,
+  noOpObjectListSetError,
   parseMustHavesBlock,
   FRONTMATTER_SCHEMAS,
   cmdFrontmatterGet,