Add terminal-native input navigation and editing

[Copilot]

The custom input handler only supported append-only text entry with backspace deletion and history navigation. No cursor positioning, word-level navigation, or standard terminal editing shortcuts.

Implementation

Cursor tracking

• Added cursor_position state variable
• Implements character insertion at cursor (not just append)
• All edit operations maintain correct cursor position

Navigation

• Character: Left/Right arrows, Home/End, Ctrl+A/E
• Word: Ctrl+Left/Right, Alt+B/F (Emacs-style)
• Helper functions find_prev_word_start() and find_next_word_start() for word boundary detection

Editing

• Delete key
• Ctrl+W (delete word backward)
• Ctrl+K (kill to end of line)
• Ctrl+U (kill to start of line)
• Ctrl+L (clear screen)

Cursor movement helper

• move_cursor_to() handles both forward and backward movement
• Correctly handles newlines in both directions (MoveUp/MoveDown + column positioning)
• All operations use this helper for consistency

Technical notes

Multi-line input support required special handling when moving cursor backward across newlines - must calculate previous line length to set correct column position. All manual MoveLeft loops replaced with the helper to avoid cursor desync.

// Before: only append at end
buffer.push(c);
print!("{}", c);

// After: insert at cursor with proper redraw
let chars: Vec<char> = buffer.chars().collect();
let before: String = chars.iter().take(cursor_position).collect();
let after: String = chars.iter().skip(cursor_position).collect();
buffer = format!("{}{}{}", before, c, after);
print!("{}", c);
if !after.is_empty() {
    print!("{}", after.replace('\n', "\r\n"));
    self.move_cursor_to(&after, after.chars().count(), 0)?;
}
cursor_position += 1;

Original prompt

Inprove the input.rs, and make it feel like terminal native way to navigate while giving inputs

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