Rapid Diffs feature parity completeness
## Context
Some MR features aren't fully parity yet when using Rapid Diffs — things like file-by-file mode, mark as viewed, hide all comments, and expand full file.
We are working towards maturing Rapid Diffs via this epic.
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## Release Notes
Rapid Diffs now supports the merge request diff features you've come to rely on — file-by-file mode, mark as viewed, hide all comments, and expand full file — so switching to the faster Rapid Diffs experience no longer means giving up the diff controls you use every day. The result is a single diff experience that's both faster and complete.
epic