Future architecture for multiple Picture-in-Picture windows

[marcoscaceres]

Follow-up from PR #245 discussions about supporting multiple PiP windows per top-level browsing context.

Key architectural ideas discussed:

List-based approach

@theIDinside proposed in this comment:

"So to actually support multiple PIP windows, we need a list of `pictureInPictureElement`s, similarly to how fullscreen does..."

And later refined to use top-level navigable scope.

Individual exit methods

@theIDinside also suggested:

"1. The individual PIP Window instances would have an `exit()` method"

Current constraint decision

@marcoscaceres noted the current browser reality in this comment:

"Can we just say that there can only be one PiP per top-level browsing context (including all child navigable, including third-party iframes). Then we don't need to deal with multiple pip directly... I checked in Safari and Chrome, it's one pip window per browser!"

For now, the spec enforces one PiP per top-level browsing context to match current browser implementations. This issue tracks the future architectural work needed if we decide to support multiple windows.