ext/standard: Optimize str_pad() using doubling copies (#23661)

This PR optimizes the user-visible `str_pad()` by reducing repeated small
copy operations during padding. In `php_str_pad_fill()`, the algorithm now uses
doubling copies to grow the written region exponentially. This lowers memcpy counts
and improves throughput for large repeated-pattern padding workloads.

Added `str_pad_repeated_pattern.phpt` to validate repeated pattern and partial-tail
boundaries, including multi-byte patterns and all pad directions. Behavior is
unchanged; this is a performance-only change and is now recorded in the PHP 8.6
performance changelog.

Author: LamentXU123

UPGRADING

@@ -1105,6 +1105,7 @@ PHP 8.6 UPGRADE NOTES
   . Improved performance of array_walk().
   . Improved performance of intval('+0b...', 2) and intval('0b...', 2).
   . Improved performance of str_split().
+  . Improved performance of str_pad().
 
 - URI:
   . Improved performance of Uri\WhatWg\Url::parse() when collecting

ext/standard/string.c

@@ -5891,13 +5891,15 @@ static void php_str_pad_fill(zend_string *result, size_t pad_chars, const char *
 		return;
 	}
 
+	const char *start = p;
 	const char *end = p + pad_chars;
-	while (p + pad_str_len <= end) {
-		p = zend_mempcpy(p, pad_str, pad_str_len);
-	}
+	size_t len = MIN(pad_str_len, pad_chars);
+	p = zend_mempcpy(p, pad_str, len);
 
-	if (p < end) {
-		memcpy(p, pad_str, end - p);
+	/* Double the filled area on each iteration. */
+	while (p < end) {
+		len = MIN(p - start, end - p);
+		p = zend_mempcpy(p, start, len);
 	}
 
 	ZSTR_LEN(result) += pad_chars;