fix(docs/manualtools.py): use removeprefix instead of lstrip for frontmatter parsing

Replace .lstrip('\n') with .removeprefix("\n") to preserve body text that
legitimately starts with blank lines. The serialize() function inserts exactly
one separator newline; removing only that one newline (via removeprefix) rather
than all leading newlines (via lstrip) maintains parse/serialize roundtrip
losslessness.

Adds regression test to verify bodies with leading blank lines roundtrip
correctly.

docs/manualtools.py

@@ -29,7 +29,7 @@ def parse(path: Path) -> tuple[dict, str]:
     if end == -1:
         return {}, text.rstrip()
     fm = yaml.safe_load(text[4:end]) or {}
-    return fm, text[end + 5 :].lstrip('\n').rstrip()
+    return fm, text[end + 5 :].removeprefix("\n").rstrip()
 
 
 def serialize(fm: dict, body: str) -> str:

docs/verify-manual.py

@@ -72,6 +72,18 @@ def test_walk_nests_directory_after_its_index():
     assert got == expected, f"wrong nesting: {got}"
 
 
+def test_parse_roundtrip_body_with_leading_blank_line():
+    fm = {"title": "Test"}
+    body = "\nContent starts after blank line."
+    text = mt.serialize(fm, body)
+    with tempfile.TemporaryDirectory() as d:
+        p = Path(d) / "t.md"
+        p.write_text(text)
+        got_fm, got_body = mt.parse(p)
+    assert got_fm == fm, f"frontmatter mismatch: {got_fm!r}"
+    assert got_body == body, f"body mismatch: expected {body!r}, got {got_body!r}"
+
+
 TESTS = [v for k, v in sorted(globals().items()) if k.startswith("test_")]