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/visitors/mrow_visitor.py

@@ -1633,6 +1633,18 @@ class MrowVisitor(Visitor):
                 # Found a fragmented function - add it to parts
                 parts.append(function_name)
                 i += consumed_count
+            elif self._is_plain_single_letter_mi(children[i]):
+                # Consecutive single-letter identifiers are joined without spaces:
+                # Word often splits a multi-letter identifier (e.g. line segment "BC")
+                # into several runs, which OMML2MML turns into adjacent <mi> elements.
+                # Re-merge them so "BC" stays "BC" instead of "B C" (which renders as B×C).
+                merged = self._convert_child(children[i])
+                j = i + 1
+                while j < len(children) and self._is_plain_single_letter_mi(children[j]):
+                    merged += self._convert_child(children[j])
+                    j += 1
+                parts.append(merged)
+                i = j
             else:
                 # No fragmented function - convert child normally
                 part = self._convert_child(children[i])
@@ -1642,6 +1654,21 @@ class MrowVisitor(Visitor):
 
         return ' '.join(parts).strip()
 
+    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 split into adjacent <mi> runs. Only plain italic identifiers are
+        merged; elements carrying an explicit mathvariant (e.g. normal for
+        chemical formulas, double-struck, etc.) are left untouched.
+        """
+        if _local(node.tag) != 'mi':
+            return False
+        if node.get('mathvariant') is not None:
+            return False
+        content = _text_content(node).strip()
+        return len(content) == 1 and content.isalpha()
+
     def _convert_children_no_space(self, node: ET.Element) -> str:
         """Convert all children and join without spaces."""
         parts = []