fix(omml2latex): 修复多字母标识符被拆开的问题

OMML2MML 会把 Word 里连写的多字母标识符（线段名 BC、三角形 △ABC、
角度 45° 等）拆成相邻的 <mi>/<mn> 元素，原转换器用空格 join 导致
'BC' 被渲染成 'B C'（B×C），'45°' 变成 '4 5°'。

三处修复：
- mrow_visitor: 连续单字母 <mi> 拼接
- legacy_adapter: <math> 根节点多子元素时连续单字母 <mi> 拼接
- math_strategy: Unicode+字母混合内容按'LaTeX命令后跟字母才加空格'规则 join

回归验证：test_mixed.docx 130 个 OMML 公式，16 个发生变化且全部为
正确改善（线段名/角度/三角形/隐式乘法拼接），零误伤。

scripts/omml_mathml2latex/compat/legacy_adapter.py

@@ -326,6 +326,19 @@ class LegacyMathMLToTeXConverter:
                         # Found fragmented function! Add it as \funcname
                         parts.append(f"\\{function_name}")
                         i += consumed  # Skip the consumed elements
+                    elif self._is_plain_single_letter_mi(children[i]):
+                        # Consecutive single-letter identifiers are joined without spaces:
+                        # Word often stores a multi-letter identifier (line segment "BC")
+                        # as a single <m:t> that OMML2MML splits into adjacent <mi> runs.
+                        # Re-merge them so "BC" stays "BC" instead of "B C" (B×C).
+                        merged = self._convert_node(children[i])
+                        j = i + 1
+                        while j < len(children) and self._is_plain_single_letter_mi(children[j]):
+                            merged += self._convert_node(children[j])
+                            j += 1
+                        if merged.strip() not in ['', '\\']:
+                            parts.append(merged)
+                        i = j
                     else:
                         # Normal element - convert and add
                         part = self._convert_node(children[i])
@@ -804,6 +817,24 @@ class LegacyMathMLToTeXConverter:
         chemical_indicators = mathrm_count + number_count
         return chemical_indicators > len(parts) / 2
 
+    def _is_plain_single_letter_mi(self, node: ET.Element) -> bool:
+        """Return True if node is a single plain-letter <mi> identifier.
+
+        Used to re-merge multi-letter identifiers (e.g. line segment "BC") that
+        Word/OMML2MML split into adjacent <mi> runs. Only plain italic identifiers
+        are merged; elements carrying an explicit mathvariant (e.g. normal for
+        chemical formulas) are left untouched.
+        """
+        if not isinstance(getattr(node, 'tag', None), str):
+            return False
+        if self._get_local_name(node.tag) != 'mi':
+            return False
+        if node.get('mathvariant') is not None:
+            return False
+        from ..utils import text_content
+        content = text_content(node).strip()
+        return len(content) == 1 and content.isalpha()
+
     def _get_local_name(self, tag: str) -> str:
         """
         Extract local element name from tag (strip namespace).