Keep @page declarations that follow a margin box

[jhaygood86]

Problem

Inside @page, any declaration that comes after a margin box is silently dropped:

@page { @top-center { content: "h" } margin: 1cm }   /* margin: 1cm lost */
@page { @top-left {…} @bottom-right {…} margin: 2cm }  /* margin: 2cm lost */

A margin box (@top-center, @bottom-right, …, per CSS Paged Media 3 §6) is parsed by CreateMarginStyle, whose inner FillDeclarations consumes the box's body through its closing }. But CreateMarginStyle takes the outer token by ref and leaves it pointing at the box's own { (where CreateMarginSelector left it). FillDeclarations then did:

var marginStyle = CreateMarginStyle(ref marginToken);
parentPageRule.AppendChild(marginStyle);
token = marginToken;   // <-- stale: the box's '{', already consumed

so the loop re-entered on a stale token and skipped past everything up to the @page block's own } — dropping any trailing declarations and any further margin boxes.

A margin box that appears last in the block (the shape the existing StyleSheetPageAtRulesAndProperties test uses) is unaffected, since there's nothing after it — which is why this went unnoticed.

Fix

Advance with NextToken() to the next real token after the consumed margin box. Declarations before a margin box already worked and are unchanged.

Tests

3 tests in Cases.cs: a declaration after one margin box, declarations both before and after, and two margin boxes followed by a declaration — asserting both the margin rules and the trailing longhands. All 3 fail on master. The existing @page test (margin box last) stays green, along with the other 1262 tests, and all seven target frameworks build with no new warnings.