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>

sks/document/document_html.cpp

@@ -1,3 +1,4 @@
+#include <iomanip>
 #include "util.h"
 #include "error.h"
 #include "log.h"
@@ -118,6 +119,24 @@ std::string Document_class::font_definitions()
             ss << "    --sans-serif: \"" << m_resolved_sans.family_name << "\", sans-serif;\n";
         if (!m_mono_typeface.empty())
             ss << "    --monospace: \"" << m_resolved_mono.family_name << "\", monospace;\n";
+        // The factor a @typeface span's font-size is multiplied by (2026-09-18).
+        // font.css renders the body at "font-size-adjust: ex-height 0.5", so
+        // the text the reader sees is 0.5 / x-height times its nominal size
+        // (EB Garamond, x-height 0.400: 1.25).  A @typeface span switches
+        // the adjust OFF (a script with no x-height cannot be matched), so
+        // its ":scale N" was N times the NOMINAL size -- about a fifth
+        // smaller against the visible text than the same N in pdf, where
+        // fontspec's Scale= is against text rendered at nominal size.
+        // Folding the body's enlargement in here makes :scale mean the
+        // same thing in both targets.  Emitted as a custom property the
+        // span multiplies in with calc(), so a fragment rendered without
+        // @document falls back to 1.  SYNC: the anchor 0.5 is font.css's.
+        const float xheight_anchor = 0.5f;
+        if (m_resolved_serif.xheight_ratio > 0.0f) {
+            std::stringstream factor {};
+            factor << std::setprecision(4) << xheight_anchor / m_resolved_serif.xheight_ratio;
+            ss << "    --typeface_scale: " << factor.str() << ";\n";
+        }
         ss << "}\n";
     }
     // Font-size normalization across families is no longer emitted from