Avoid redundant whitespace scanning in magic comment lexing

Author: kddnewton

src/prism.c

@@ -7587,11 +7587,17 @@ parser_lex_magic_comment(pm_parser_t *parser, bool semantic_token_seen) {
         if (pm_memchr(start, ':', (size_t) (end - start), parser->encoding_changed, parser->encoding) == NULL) {
             return false;
         }
+
+        // Advance start past leading whitespace so the main loop begins
+        // directly at the key, avoiding a redundant whitespace scan.
+        start += pm_strspn_whitespace(start, end - start);
     }
 
     cursor = start;
     while (cursor < end) {
-        while (cursor < end && (pm_char_is_magic_comment_key_delimiter(*cursor) || pm_char_is_whitespace(*cursor))) cursor++;
+        if (indicator) {
+            cursor += pm_strspn_whitespace(cursor, end - cursor);
+        }
 
         const uint8_t *key_start = cursor;
         while (cursor < end && (!pm_char_is_magic_comment_key_delimiter(*cursor) && !pm_char_is_whitespace(*cursor))) cursor++;