txt section numbering, @typeface :middle/:lift and html scale, NewGardinerOmni, html :leading (from dev 9fa469dd74ed)

Section headings under txt are numbered inside a @document only; @typeface
gains :middle and :lift and its :scale now means the same in html and
pdf; NewGardinerOmni ships (OFL 1.1, Mark-Jan Nederhof) and @egyptian /
@chinese replace @cjk; @table :leading and :colsep reach html; kdesc
--typeface help names the flag; @document.txt is a Python class.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

mac/font_store.cpp

@@ -175,6 +175,15 @@ strings_t available_font_families()
     return result;
 }
 
+bool font_installed(const std::string& name)
+{
+    const std::string wanted = name_to_dirname(name);
+    for (const std::string& family : available_font_families())
+        if (name_to_dirname(family) == wanted)
+            return true;
+    return false;
+}
+
 
 static void extract_font_metrics(Resolved_font& font)
 {