LaTeX reader: peek at next token directly in peekTok.

peekTok went through lookAhead (satisfyTok (const True)), paying
for tokenPrim position bookkeeping, the raw-token state update, and
lookAhead state save/restore on every call; peekTok is used by the
main inline and block dispatchers for every element.  Just examine
the head of the input stream after expanding macros.

Co-Authored-By: Claude <noreply@anthropic.com>

src/Text/Pandoc/Readers/LaTeX/Parsing.hs

@@ -579,7 +579,10 @@ satisfyTok f = do
 peekTok :: PandocMonad m => LP m Tok
 peekTok = do
   doMacros
-  lookAhead (satisfyTok (const True))
+  TokStream _ toks <- getInput
+  case toks of
+    t : _ -> return t
+    []    -> mzero
 
 doMacros :: PandocMonad m => LP m ()
 doMacros = do