This fixes an oversight from #42783: when detecting that a box can be reused, we weren't clearing the box damage propagated to ancestors, so ancestors were still recollecting children.

This exposed some problems in rebuild_box_tree_from_independent_formatting_context():

1. When reusing a block-level box that established an independent formatting context, we weren't checking that the new style was still block-level.
2. When reusing a block-level abspos box, we weren't checking if the new original display was inline-level, preventing it from being handled as a block-level because of the static position.
3. When reusing a flex item, we weren't checking if the order had changed.

Testing: we have test coverage for the 3 problems above:

1. Covered by /html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/legend-display.html, and also adding a new /css/css-display/display-flow-root-dynamic.html.
2. Covered by /css/CSS2/abspos/hypothetical-box-dynamic.html
3. Covered by /css/css-display/order-dynamic.html.
   Reviewed in layout: Remove box damage for parent when we can reuse the box servo/servo#42847