fix: YAML-parity title edges — inline comments, colon-space mapping rule

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

Author: cgreeno

pkg/diagram/frontmatter.go

@@ -33,10 +33,18 @@ func StripFrontmatter(input string) (rest string, title string) {
 			return strings.Join(lines[i+1:], "\n"), title
 		}
 		// Only a top-level `title:` key counts — indented occurrences are
-		// nested config values (e.g. inside themeCSS), not the title.
+		// nested config values (e.g. inside themeCSS), not the title. YAML
+		// requires whitespace after the colon for a mapping ("title:xyz" is a
+		// plain scalar, not a key), and an unquoted value ends at a comment.
 		trimmed := strings.TrimRight(lines[i], " \t\r")
-		if v, ok := strings.CutPrefix(trimmed, indent+"title:"); ok {
-			title = strings.Trim(strings.TrimSpace(v), `"'`)
+		if v, ok := strings.CutPrefix(trimmed, indent+"title:"); ok && (v == "" || v[0] == ' ' || v[0] == '\t') {
+			v = strings.TrimSpace(v)
+			if !strings.HasPrefix(v, `"`) && !strings.HasPrefix(v, `'`) {
+				if idx := strings.Index(v, " #"); idx != -1 {
+					v = strings.TrimSpace(v[:idx])
+				}
+			}
+			title = strings.Trim(v, `"'`)
 		}
 	}
 	return input, ""

pkg/diagram/frontmatter_test.go

@@ -25,11 +25,14 @@ func TestStripFrontmatter(t *testing.T) {
 		{"indented title is not the title", "---\nconfig:\n  title: nested\n---\ndiagram", "diagram", ""},
 		{"crlf input", "---\r\ntitle: t\r\n---\r\ndiagram", "diagram", "t"},
 		{"multiline themeCSS config", "---\nconfig:\n  themeCSS: |\n    rect { fill: red; }\n---\nerDiagram", "erDiagram", ""},
+		{"inline comment stripped from title", "---\ntitle: hi # note\n---\ndiagram", "diagram", "hi"},
+		{"hash kept inside quoted title", "---\ntitle: \"a # b\"\n---\ndiagram", "diagram", "a # b"},
+		{"no space after colon is not a mapping", "---\ntitle:xyz\n---\ndiagram", "diagram", ""},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {
 			rest, title := StripFrontmatter(c.in)
-			if rest != c.wantRest && normalize(rest) != c.wantRest {
+			if rest != c.wantRest {
 				t.Errorf("rest = %q, want %q", rest, c.wantRest)
 			}
 			if title != c.wantTitle {
@@ -38,14 +41,3 @@ func TestStripFrontmatter(t *testing.T) {
 		})
 	}
 }
-
-// normalize strips \r so CRLF inputs compare against \n expectations.
-func normalize(s string) string {
-	out := make([]rune, 0, len(s))
-	for _, r := range s {
-		if r != '\r' {
-			out = append(out, r)
-		}
-	}
-	return string(out)
-}