YAML frontmatter: drop block scalars so all strings round-trip

Block scalars cannot preserve newline-only strings on Python 3.12
where splitlines() yields ['']. Always emit double-quoted scalars.
Widen test_escape_yaml_roundtrip to cover multiline text.

Author: clean6378-max-it

tests/test_md_exporter_yaml.py

@@ -82,10 +82,10 @@ def test_yaml_frontmatter_roundtrip(self):
         assert yaml.safe_load(md.split("---")[1]) == _session_frontmatter_dict(session)
         assert _extract_frontmatter_dict(md) == _session_frontmatter_dict(session)
 
-    def test_multiline_title_uses_block_scalar(self):
+    def test_multiline_title_uses_quoted_scalar(self):
         session = _base_session(title="line one\nline two")
         md = session_to_markdown(session)
-        assert "title: |-" in md.split("---")[1]
+        assert 'title: "line one\\nline two"' in md.split("---")[1]
         assert _extract_frontmatter_dict(md)["title"] == "line one\nline two"
 
     def test_tab_and_hash_in_title(self):
@@ -97,9 +97,7 @@ def test_tab_and_hash_in_title(self):
 @FUZZ_SETTINGS
 @given(st.text())
 def test_escape_yaml_roundtrip(s: str) -> None:
-    """Double-quoted scalars round-trip for arbitrary single-line text."""
-    if "\n" in s or "\r" in s:
-        return
+    """Double-quoted scalars round-trip for arbitrary text."""
     loaded = yaml.safe_load(f"key: {_escape_yaml(s)}")
     assert loaded["key"] == s
 

utils/md_exporter.py

@@ -478,7 +478,7 @@ def _yaml_mapping_key(key: str) -> str:
 
 
 def _escape_yaml(s: str) -> str:
-    """Return a YAML double-quoted scalar for a single-line string value."""
+    """Return a YAML double-quoted scalar for any string (including embedded newlines)."""
     parts: list[str] = []
     for ch in s:
         if ch == "\\":
@@ -522,15 +522,6 @@ def _append_yaml_value(lines: list[str], key: str, value: Any, *, indent: int =
         return
     if not isinstance(value, str):
         raise TypeError(f"unsupported frontmatter value type: {type(value).__name__}")
-    if "\n" in value:
-        content_lines = value.splitlines()
-        if content_lines and all(ch in "\n\r\t" or ch.isprintable() for ch in value):
-            lines.append(f"{prefix}{key}: |-")
-            for line in content_lines:
-                lines.append(f"{prefix}  {line}")
-            return
-        lines.append(f"{prefix}{key}: {_escape_yaml(value)}")
-        return
     lines.append(f"{prefix}{key}: {_escape_yaml(value)}")