[Float][Fiber] Assume stylesheets in document are already loaded (#29811) #29812
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Cherry pick of #29811
When we made stylesheets suspend even during high priority updates we exposed a bug in the loading tracking of stylesheets that are loaded as part of the preamble. This allowed these stylesheets to put suspense boundaries into fallback mode more often than expected because cases where a stylesheet was server rendered could now cause a fallback to trigger which was never intended to happen.
This fix updates resource construction to evaluate whether the instance exists in the DOM prior to construction and if so marks the resource as loaded and inserted.
One ambiguity that needed to be solved still is how to tell whether a stylesheet rendered as part of a late Suspense boundary reveal is already loaded. I updated the instruction to clear out the loading promise after successfully loading. This is useful because later if we encounter this same resource again we can avoid the microtask if it is already loaded. It also means that we can concretely understand that if a stylesheet is in the DOM without this marker then it must have loaded (or errored) already.