Revert truncatedDiff limit back to 80 lines

The debug increase to 300 lines confirmed that the DatePeriod serialize
test failures are due to content mismatches beyond line 80, not EXPECTF
regex issues. Reverting to keep CI output manageable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MagicalTux

php_test.go

@@ -69,7 +69,7 @@ func (lb *limitedBuffer) String() string { return lb.buf.String() }
 // truncatedDiff computes a diff but truncates inputs to avoid O(n²) blowup
 // on large outputs with many differences.
 func truncatedDiff(expected, actual string) string {
-	const maxLines = 300
+	const maxLines = 80
 	truncate := func(s string) string {
 		lines := strings.SplitN(s, "\n", maxLines+1)
 		if len(lines) > maxLines {