fix(markdown): MULTI_FRONTMATTER stops at the first non-frontmatter line

Review catch on #3163: the fence-pairing scan walked arbitrarily far past
body prose to find a second ---, and one colon-prefixed prose line
(`Note: …`, `TODO: …`) was enough to reject legitimate content at
import. Issue #2743's spec is leading-only detection that stops at the
first non-frontmatter character — the walk now breaks on the first line
that isn't YAML-shaped (key:, list item, comment, indented continuation,
or blank), so an hrule followed by prose is body content, never a
stacked block. Real stacked blocks (adjacent, blank-separated,
list-valued keys) still reject.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/core/markdown.ts

@@ -338,17 +338,31 @@ function collectValidationErrors(
   //    already-serialized markdown re-wrapped in fresh frontmatter).
   //    gray-matter parses only the first block and silently leaves the second
   //    in the body. Heuristic: first non-empty line after the close is `---`,
-  //    a later `---` closes it, and at least one line between looks like a
-  //    YAML `key:` — a lone `---` stays a markdown horizontal rule.
+  //    a later `---` closes it, EVERY line between is frontmatter-shaped
+  //    (YAML `key:`, `- ` list item, `#` comment, indented continuation, or
+  //    blank — the issue's "stop at the first non-frontmatter character"
+  //    spec), and at least one is a `key:` line. A lone `---` stays a
+  //    markdown horizontal rule, and an hrule followed by prose — even
+  //    colon-prefixed prose like `Note: …` mixed with plain lines — is body
+  //    content, not a stacked block.
   let afterClose = closeLine + 1;
   while (afterClose < lines.length && lines[afterClose].trim().length === 0) afterClose++;
   if (afterClose < lines.length && lines[afterClose].trim() === '---') {
     let secondClose = -1;
     for (let i = afterClose + 1; i < lines.length; i++) {
-      if (lines[i].trim() === '---') {
+      const trimmed = lines[i].trim();
+      if (trimmed === '---') {
         secondClose = i;
         break;
       }
+      const yamlShaped =
+        trimmed.length === 0 ||
+        /^[A-Za-z_][\w-]*\s*:/.test(trimmed) ||
+        trimmed.startsWith('- ') ||
+        trimmed === '-' ||
+        trimmed.startsWith('#') ||
+        /^\s/.test(lines[i]);
+      if (!yamlShaped) break; // first non-frontmatter line → body prose, not a stacked block
     }
     if (
       secondClose > afterClose + 1 &&

test/markdown-validation.test.ts

@@ -286,6 +286,24 @@ body`;
       const parsed = parseMarkdown(md, undefined, { validate: true });
       expect(parsed.errors!.map(e => e.code)).not.toContain('MULTI_FRONTMATTER');
     });
+
+    test('body hrule + colon-prefixed prose (`Note: …`) mixed with plain lines is NOT flagged', () => {
+      const md = `${fence}\ntitle: ok\ntype: concept\n${fence}\n\n${fence}\n\nNote: remember to follow up\n\nlots of plain prose here\n\n${fence}\n\nmore prose`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).not.toContain('MULTI_FRONTMATTER');
+    });
+
+    test('fence pairing stops at the first non-frontmatter line (no far-fence pairing across prose)', () => {
+      const md = `${fence}\ntitle: ok\n${fence}\n\n${fence}\n\n${'plain prose line\n'.repeat(40)}TODO: fix the widget\n${'more prose\n'.repeat(40)}${fence}\nend`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).not.toContain('MULTI_FRONTMATTER');
+    });
+
+    test('stacked block with list-valued keys is still flagged', () => {
+      const md = `${fence}\ntitle: outer\n${fence}\n\n${fence}\ntitle: inner\ntags:\n  - a\n  - b\n${fence}\n\nbody`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).toContain('MULTI_FRONTMATTER');
+    });
   });
 
   test('error.line is set for line-bearing errors', () => {