[Story Player] Load player iframes sequentially when starting from other than first story

[mszylkowski]

Describe the new feature or change to an existing feature you'd like to see

The player loads too many stories at once and there is code to optimize the loading for the first story, but if you start with the second Story it takes forever to load.

We need to change the logic from:
load the current story as visible + 2 others as prerender
to:
load the current story as visible, and once it's done, load the next one as prerender

Additional context

Publishers load a story using show()

• in that show() method, there's this line that finds the adjascent stories
• And this line will start the logic that will position + preload the stories
• the story will prerender, or load, when you call this line here
• it's gated behind navigationPromise that's a deferred that waits for the visible story to send its storyContentLoaded event

Related to #31562, #31925

[gmajoulet]

Crossposting:

Objective

Improve LCP of the visible Story when using the show() method on the non first Story.
Calling show() to display the n-th story should be just as fast as displaying the first Story.

How

Loading a Story is using quite some bandwidth, and loading several at once introduces significant delays on the LCP of the visible one.
We should always:

1. Load the visible story, and wait for storyContentLoaded event, then
2. Load the N + 1 story with a prerender visibilitystate. No need to prerender the previous one until user tries swiping to it

[gmajoulet]

cc @ampproject/wg-stories