fix(omml2latex): fragmented-function 前缀匹配，修复 sinx 误输出

mrow_visitor 的 _check_fragmented_function_at_position 把所有连续单字母
拼成整串后只查整串是否在 MATH_FUNCTIONS，导致 <mi>s</mi><mi>i</mi><mi>n</mi>
<mi>x</mi> 输出 'sinx' 而非 '\sin x'。

改为前缀匹配（对齐 legacy_adapter._check_fragmented_function_at_level），
从最长到最短找已知函数名，只消耗函数名对应字母，其余留给调用方。

验证：sinx/cosx/tanx → \sin x/\cos x/\tan x；纯 sin/log、log_2(msub)、
BC 拼接、abc 非函数等场景零回归；test_mixed.docx 130 公式输出不变。

scripts/omml_mathml2latex/visitors/mrow_visitor.py

@@ -546,14 +546,28 @@ class MrowVisitor(Visitor):
         if len(letters) < 2:
             return ("", 0, None)
 
-        # Check if the sequence forms a known function
+        # Check if the sequence forms a known function.
+        # Use prefix matching so that "sinx" resolves to "sin" (leaving the
+        # trailing "x" for the caller to convert), rather than requiring the
+        # whole run to be a function name. Mirrors the logic in
+        # legacy_adapter._check_fragmented_function_at_level.
         from ..data import MATH_FUNCTIONS
-        potential_function = ''.join(letters)
+        potential_function = ""
+        matched_length = 0
+        for length in range(min(len(letters), 6), 1, -1):
+            candidate = ''.join(letters[:length])
+            if candidate in MATH_FUNCTIONS:
+                potential_function = candidate
+                matched_length = length
+                break
 
-
+        if not potential_function:
-        if potential_function not in MATH_FUNCTIONS:
             return ("", 0, None)
 
+        # Only the matched letters belong to the function; any trailing letters
+        # (e.g. the "x" in "sinx") remain for the caller to convert.
+        consumed_count = matched_length
+
         # Build the function LaTeX
         function_latex = f"\\{potential_function}"