Date/Time: Clamp the start of the week in get_calendar().

The `start_of_week` option is cast to an int but otherwise unchecked, so a
negative value makes `( $wdcount + $week_begins ) % 7` negative, and a value
above 6 is out of range for `WP_Locale::get_weekday()`. Either leads to an
undefined array key. Clamping it to 0-6 keeps the weekday headers valid and
satisfies the new `int<0, 6>` type of `WP_Locale::get_weekday()`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: westonruter

src/wp-includes/general-template.php

@@ -2666,7 +2666,7 @@ function get_calendar( $args = array() ) {
 	}
 
 	// week_begins = 0 stands for Sunday.
-	$week_begins = (int) get_option( 'start_of_week' );
+	$week_begins = clamp( (int) get_option( 'start_of_week' ), 0, 6 );
 
 	// Read the current date.
 	list( $current_year, $current_month, $current_day ) = array_map( 'intval', explode( '-', current_time( 'Y-m-j' ) ) );