Unroll whitespace check in '--' line-comment dispatch

The leading-whitespace skip at the top of read_next_token() was
already unrolled into byte-equality checks for the perf reasons
documented in 916b512. Apply the same unroll to the third-byte
whitespace check that gates a '--' as a line-comment start, so the
hot dispatch chain doesn't fall back into strpos() on a 5-char mask
for this case. The bound check is folded into '?? null' on the
third-byte read, matching the rest of the lookahead style.

Author: JanJakes

packages/mysql-on-sqlite/src/mysql/class-wp-mysql-lexer.php

@@ -2507,10 +2507,16 @@ private function read_next_token(): ?int {
 				$type = self::LOGICAL_NOT_OPERATOR;
 			}
 		} elseif ( '-' === $byte ) {
+			$third_byte = $this->sql[ $this->bytes_already_read + 2 ] ?? null;
 			if (
 				'-' === $next_byte
-				&& $this->bytes_already_read + 2 < $this->sql_length
-				&& false !== strpos( self::WHITESPACE_MASK, $this->sql[ $this->bytes_already_read + 2 ] )
+				&& (
+					' ' === $third_byte
+					|| "\t" === $third_byte
+					|| "\n" === $third_byte
+					|| "\r" === $third_byte
+					|| "\f" === $third_byte
+				)
 			) {
 				$type = $this->read_line_comment();
 			} elseif ( '>' === $next_byte ) {