Add rprompt support for right-side prompt display

[amatsuda]

Summary

This patch adds support for displaying a right-aligned prompt (rprompt) similar to zsh's RPROMPT feature.

• The rprompt is displayed at the right edge of the terminal
• Automatically hides when the input line gets too long (prevents overlap)
• Persists during line editing (backspace, Ctrl-U, etc.) because it's part of Reline's render cycle

Usage

Reline.rprompt = "[%H:%M]"
Reline.readline("> ")

Implementation

• Added rprompt attribute to Reline::Core (reader + setter)
• Pass rprompt to LineEditor in inner_readline
• In LineEditor#render, add rprompt as a segment on the first visible line at [right_col, width, text]
• Uses the same segment-based rendering as dialogs

Motivation

I wanted to have this feature for my app that I'm building on top of Reline, and found it difficult to implement without patching Reline itself.

Testing

Behavior of the RPROMPT feature can be confirmed with something like this.

ruby -rreline -Ilib -e 'Reline.rprompt = "[#{Dir.pwd}]"; print Reline.readline("> ")'