Render differential updates by named segments instead of array index

[tompng]

Summary

render_line_differential matches the segments of each rendered line between the previous and the current frame to compute a minimal screen update. This matching used to be positional: a line was an array of [x, width, content] tuples where the array index doubled as the segment's identity (index 0 = prompt, 1 = input line, 2 = rprompt, 3.. = dialogs). Keeping identities stable required nil-padding and magic index offsets like dialog_rows[index + 3] = ..., which is fragile and hard to extend when adding a new kind of segment.

This PR gives each segment an explicit identifier and changes the tuple to [identifier, x, width, content]:

• Built-in segments: :builtin_prompt, :builtin_line, :builtin_rprompt, :builtin_menu
• Dialogs: :"dialog_#{dialog.name}" — the dialog_ prefix isolates user-provided dialog names from the builtin identifiers and from the internal markers (:skip, :blank, :outdated)

Idea for the future

I have an idea to add a completion preview if there is only one candidate, or there's a common prefix.
This PR will be a preparation for such change.