OMML/MathML writers: Use upright font for numbers by default

Fixes rendering of numbers in OMML and MathML output to use upright
(roman) font style by default, matching the behavior of other LaTeX
engines and typographic standards.

## Background and Rationale

According to typographic standards and mathematical typesetting
conventions:

1. **ISO 80000-2 Standard** (Quantities and units — Part 2: Mathematics)
   specifies that numbers should be set in upright (roman) font.

2. **LaTeX Default Behavior**: In math mode, LaTeX renders digits in
   upright font by default. The Computer Modern font family, used by
default in LaTeX, has separate italic and upright variants, and numbers
always use the upright variant in math mode.

3. **MathML Specification**: The `<mn>` element represents "numeric
   literal" content and should be rendered with upright font styling
unless explicitly overridden with mathvariant attribute.

4. **OMML Specification**: Numbers in Office Math ML should use
   m:sty="p" (plain/roman) to indicate upright font.

## What Changed

### OMML Writer (src/Text/TeXMath/Writers/OMML.hs)

- Numbers now get m:sty="p" attribute by default when no explicit style
  is set, matching the treatment of uppercase Greek letters (see #255)

### MathML Writer (src/Text/TeXMath/Writers/MathML.hs)

- `<mn>` elements now get mathvariant="normal" attribute by default when
  no explicit style is set, matching the treatment of uppercase Greek
letters (see #255)

### Test Data Fix

- Fixed test/writer/omml/simplePres.test: Changed ENumber "\946" (β) to
  EIdentifier "\946", as Greek letters should be identifiers, not
numbers

## Examples

Before:
  OMML: `<m:r><m:t>123</m:t></m:r>` (rendered as italic in Word)
  MathML: `<mn>123</mn>` (may render as italic depending on context)

After:
  OMML: `<m:r><m:rPr><m:sty m:val="p" /></m:rPr><m:t>123</m:t></m:r>`
  MathML: `<mn mathvariant="normal">123</mn>`

## Compatibility

- Explicitly styled numbers (e.g., $\mathit{123}$) still render as
  italic
- Variables remain italic
- Uppercase Greek letters remain upright (already fixed in #255)
- Lowercase Greek letters remain italic
- This aligns texmath's output with standard LaTeX/pdfLaTeX behavior

## Related

- Closes #255 (uppercase Greek letters fix)
- Similar to: Typst issue #4139 "Italic digits do not work in math"

## References

- ISO 80000-2:2019: Quantities and units — Part 2: Mathematics
  https://www.iso.org/standard/64973.html
- "Typesetting Mathematics According to the ISO Standard" by Nick Higham
  https://nhigham.com/2016/01/28/typesetting-mathematics-according-to-the-iso-standard/
- "upright italics (math mode)?" - TeX StackExchange discussion
  https://tex.stackexchange.com/questions/431740/upright-italics-math-mode
- MathML Core specification on `<mn>` element
  https://w3c.github.io/mathml-core/spec.html
- isomath package documentation (ISO 80000-2 compliant typesetting)
  https://ctan.math.utah.edu/ctan/tex-archive/macros/latex/contrib/isomath/isomath.pdf

Author: val3344

src/Text/TeXMath/Writers/MathML.hs

@@ -196,7 +196,7 @@ showExp tt e =
      vnode elname t
        = case tt of
            Nothing ->
-             if isUppercaseGreek t -- see #255
+             if elname == "mn" || isUppercaseGreek t -- see #255
                 then withAttribute "mathvariant" "normal" $ tunode elname t
                 else tunode elname t
            Just TextNormal -> withAttribute "mathvariant" "normal" $

src/Text/TeXMath/Writers/OMML.hs

@@ -206,7 +206,9 @@ handleDownup' _ xs = xs
 showExp :: [Element] -> Exp -> [Element]
 showExp props e =
  case e of
-   ENumber x        -> [str props x]
+   ENumber x
+     | null props -> [str [mnodeA "sty" "p" ()] x]
+     | otherwise  -> [str props x]
    EGrouped [EUnderover _ (ESymbol Op s) y z, w] ->
      [makeNary props "undOvr" s y z w]
    EGrouped [ESubsup (ESymbol Op s) y z, w] ->

test/writer/mml/01.test

@@ -33,17 +33,17 @@
           <mrow>
             <msup>
               <mi>b</mi>
-              <mn>2</mn>
+              <mn mathvariant="normal">2</mn>
             </msup>
             <mo>−</mo>
-            <mn>4</mn>
+            <mn mathvariant="normal">4</mn>
             <mi>a</mi>
             <mi>c</mi>
           </mrow>
         </msqrt>
       </mrow>
       <mrow>
-        <mn>2</mn>
+        <mn mathvariant="normal">2</mn>
         <mi>a</mi>
       </mrow>
     </mfrac>

test/writer/mml/02.test

@@ -25,24 +25,24 @@
 <?xml version='1.0' ?>
 <math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
   <mrow>
-    <mn>2</mn>
+    <mn mathvariant="normal">2</mn>
     <mo>=</mo>
     <mrow>
       <mo stretchy="true" form="prefix">(</mo>
       <mfrac>
         <mrow>
           <mrow>
             <mo stretchy="true" form="prefix">(</mo>
-            <mn>3</mn>
+            <mn mathvariant="normal">3</mn>
             <mo>−</mo>
             <mi>x</mi>
             <mo stretchy="true" form="postfix">)</mo>
           </mrow>
           <mo>×</mo>
-          <mn>2</mn>
+          <mn mathvariant="normal">2</mn>
         </mrow>
         <mrow>
-          <mn>3</mn>
+          <mn mathvariant="normal">3</mn>
           <mo>−</mo>
           <mi>x</mi>
         </mrow>

test/writer/mml/03.test

@@ -16,14 +16,14 @@
     <mi>a</mi>
     <msup>
       <mi>x</mi>
-      <mn>2</mn>
+      <mn mathvariant="normal">2</mn>
     </msup>
     <mo>+</mo>
     <mi>b</mi>
     <mi>x</mi>
     <mo>+</mo>
     <mi>c</mi>
     <mo>=</mo>
-    <mn>0</mn>
+    <mn mathvariant="normal">0</mn>
   </mrow>
 </math>

test/writer/mml/04.test

@@ -34,14 +34,14 @@
       <msup>
         <mrow>
           <mo stretchy="true" form="prefix">(</mo>
-          <mn>5</mn>
+          <mn mathvariant="normal">5</mn>
           <mo>−</mo>
           <mi>T</mi>
           <mo stretchy="true" form="postfix">)</mo>
         </mrow>
-        <mn>2</mn>
+        <mn mathvariant="normal">2</mn>
       </msup>
-      <mn>2</mn>
+      <mn mathvariant="normal">2</mn>
     </mfrac>
   </mrow>
 </math>

test/writer/mml/06.test

@@ -40,7 +40,7 @@
       <mrow>
         <mi>m</mi>
         <mo>=</mo>
-        <mn>1</mn>
+        <mn mathvariant="normal">1</mn>
       </mrow>
       <mo accent="false">∞</mo>
     </munderover>
@@ -49,37 +49,37 @@
       <mrow>
         <mi>n</mi>
         <mo>=</mo>
-        <mn>1</mn>
+        <mn mathvariant="normal">1</mn>
       </mrow>
       <mo accent="false">∞</mo>
     </munderover>
     <mfrac>
       <mrow>
         <msup>
           <mi>m</mi>
-          <mn>2</mn>
+          <mn mathvariant="normal">2</mn>
         </msup>
         <mspace width="0.167em" />
         <mi>n</mi>
       </mrow>
       <mrow>
         <msup>
-          <mn>3</mn>
+          <mn mathvariant="normal">3</mn>
           <mi>m</mi>
         </msup>
         <mrow>
           <mo stretchy="true" form="prefix">(</mo>
           <mi>m</mi>
           <mspace width="0.167em" />
           <msup>
-            <mn>3</mn>
+            <mn mathvariant="normal">3</mn>
             <mi>n</mi>
           </msup>
           <mo>+</mo>
           <mi>n</mi>
           <mspace width="0.167em" />
           <msup>
-            <mn>3</mn>
+            <mn mathvariant="normal">3</mn>
             <mi>m</mi>
           </msup>
           <mo stretchy="true" form="postfix">)</mo>

test/writer/mml/09.test

@@ -25,7 +25,7 @@
         <mo>→</mo>
         <msub>
           <mi>z</mi>
-          <mn>0</mn>
+          <mn mathvariant="normal">0</mn>
         </msub>
       </mrow>
     </munder>
@@ -41,7 +41,7 @@
       <mo stretchy="true" form="prefix">(</mo>
       <msub>
         <mi>z</mi>
-        <mn>0</mn>
+        <mn mathvariant="normal">0</mn>
       </msub>
       <mo stretchy="true" form="postfix">)</mo>
     </mrow>

test/writer/mml/10.test

@@ -56,22 +56,22 @@
     </mrow>
     <mo>=</mo>
     <mfrac>
-      <mn>1</mn>
+      <mn mathvariant="normal">1</mn>
       <mrow>
-        <mn>4</mn>
+        <mn mathvariant="normal">4</mn>
         <msup>
           <mi>π</mi>
-          <mn>2</mn>
+          <mn mathvariant="normal">2</mn>
         </msup>
         <msup>
           <mi>κ</mi>
-          <mn>2</mn>
+          <mn mathvariant="normal">2</mn>
         </msup>
       </mrow>
     </mfrac>
     <msubsup>
       <mo>∫</mo>
-      <mn>0</mn>
+      <mn mathvariant="normal">0</mn>
       <mi>∞</mi>
     </msubsup>
     <mfrac>
@@ -103,7 +103,7 @@
       <mo stretchy="true" form="prefix">[</mo>
       <msup>
         <mi>R</mi>
-        <mn>2</mn>
+        <mn mathvariant="normal">2</mn>
       </msup>
       <mfrac>
         <mrow>

test/writer/mml/11.test

@@ -36,38 +36,38 @@
       <mo stretchy="true" form="postfix">)</mo>
     </mrow>
     <mo>=</mo>
-    <mn>0.033</mn>
+    <mn mathvariant="normal">0.033</mn>
     <msubsup>
       <mi>C</mi>
       <mi>n</mi>
-      <mn>2</mn>
+      <mn mathvariant="normal">2</mn>
     </msubsup>
     <msup>
       <mi>κ</mi>
       <mrow>
         <mo>−</mo>
-        <mn>11</mn>
+        <mn mathvariant="normal">11</mn>
         <mi>/</mi>
-        <mn>3</mn>
+        <mn mathvariant="normal">3</mn>
       </mrow>
     </msup>
     <mo>,</mo>
     <mspace width="1.0em" />
     <mfrac>
-      <mn>1</mn>
+      <mn mathvariant="normal">1</mn>
       <msub>
         <mi>L</mi>
-        <mn>0</mn>
+        <mn mathvariant="normal">0</mn>
       </msub>
     </mfrac>
     <mo>≪</mo>
     <mi>κ</mi>
     <mo>≪</mo>
     <mfrac>
-      <mn>1</mn>
+      <mn mathvariant="normal">1</mn>
       <msub>
         <mi>l</mi>
-        <mn>0</mn>
+        <mn mathvariant="normal">0</mn>
       </msub>
     </mfrac>
   </mrow>