Toggle reviewed files with Space

[bdice]

Summary

Note: Because this PR changes the core keybinding from Escape to Space, it is not intended for upstream submission.

• support both the redesigned and classic GitHub Files changed interfaces
• use Space to toggle the hovered file's Viewed state
• collapse viewed files and advance the next rendered file to the top of the visible diff region
• expand viewed files and keep the restored file at the top of the visible diff region
• preserve normal Space behavior while typing in comments and form fields
• measure GitHub's sticky header dynamically to avoid scrolling file headers underneath it
• point userscript updates and documentation at this customized fork

Why

The original userscript relies on DOM selectors from GitHub's older review interface and an Escape handler that no longer works reliably. GitHub's redesigned interface exposes different file containers and accessible toggle state. This fork also uses a workflow optimized for reviewing many small files in sequence.

User impact

Hovering a diff and pressing Space now provides a reversible review workflow: mark and advance, or restore and refocus. The shortcut does not intercept spaces typed into editable controls.

Validation

• node --check main.js
• git diff --check