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.h

@@ -28,6 +28,12 @@ std::vector<std::string> installed_font_dirs();
 std::string default_font_dir();
 // Every installable font family found across those directories.
 std::vector<std::string> available_font_families();
+// Whether the store can resolve NAME -- written as the family name or as
+// the store's directory name, since both resolve.  The question a klammer
+// asks before it names a typeface that is not shipped (a CJK face), so it
+// can render the text plainly and say how to install the font instead of
+// failing the document.  Never throws; resolve_font() is the error path.
+bool font_installed(const std::string& name);
 // The Locator is the DOCUMENT location to blame when the typeface is not
 // installed: without it the "not installed" error cites font_store.cpp,
 // a source location no user can act on (the no-.cpp-location rule).