feat: add disableFullScreenRoute to let the embedder own fullscreen

[Ortes]

Motivation

Chewie's fullscreen is implemented by pushing a dedicated route and re-parenting the player into it. On web this reparents the platform-view <video> element; on exit the original view goes blank, and the common workaround is to re-initialize the controller — which for an HLS source means a full manifest reload and rebuffer.

Some embedders want to own the fullscreen presentation themselves (e.g. expand their own layout and drive the browser's native Fullscreen API) without Chewie's route getting in the way.

What this adds

ChewieController.disableFullScreenRoute (default false). When true:

• Toggling fullscreen no longer pushes/pops Chewie's internal route.
• isFullScreen still flips, so the fullscreen control icon updates as usual.
• The player widget stays mounted in place; the host app is responsible for the fullscreen presentation.

This sidesteps the platform-view teardown/reload entirely by never moving the element.

API

• bool disableFullScreenRoute on ChewieController (constructor + copyWith), default false.

Backwards compatibility

Fully backwards compatible — defaults to false, preserving the current route-based behavior. Opt-in only.

Testing

• dart analyze — no issues.
• In production use in an HLS web player, where it eliminates the manifest reload/rebuffer on fullscreen exit.

Complementary to the native web-fullscreen work in #946.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.66%. Comparing base (69a8fc7) to head (1c4e19a).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #947      +/-   ##
==========================================
+ Coverage   43.82%   48.66%   +4.84%     
==========================================
  Files          21       21              
  Lines        1602     1605       +3     
==========================================
+ Hits          702      781      +79     
+ Misses        900      824      -76     

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