Improve Rapid Diffs accessibility for screen reader users

What does this MR do and why?

Follows up on the screen-reader accessibility issues reported in the referenced issue. In Rapid Diffs, screen reader users could not tell whether a diff line was added, removed, or unchanged: the +/− signs were explicitly suppressed from assistive technology and the change type was only announced on the line-number link, forcing extra navigation per line.

This MR:

• restores the announced +/− line signs (matching legacy diffs, which read well with screen readers);
• shortens the line-number link labels to just the number, so the change type is carried by the sign instead of a verbose "Added line N";
• adds a per-file screen-reader-only <caption> summarizing added/removed lines;
• marks the diff table column headers with a scope.

How to reproduce

1. Turn on a screen reader (NVDA + Firefox, or VoiceOver + Safari).
2. Open a merge request and enable Rapid Diffs.
3. Navigate through the diff lines and listen to how each line and line number are announced.

Screen-reader verification

VoiceOver walkthrough on Rapid Diffs — for each line the change type (“plus” / “minus” / no sign) and its line number(s) are announced, followed by the per-file summary caption:

References

• #451851