fix(themes/powerline): ensure prompt starts on a new line

When a command's output has no trailing newline, \e[G (column 1) in PS1
would overwrite that output with the prompt. Add a missing-newline
indicator in PROMPT_COMMAND: reversed-video % marks the incomplete line,
then \r\e[K moves to column 1 and clears it, so the prompt always
renders cleanly regardless of the previous command's output.

Fixes #2372

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: seefood

themes/powerline/powerline.base.bash

@@ -290,6 +290,10 @@ function __powerline_prompt_command() {
 	local beginning_of_line='\[\e[G\]'
 	local info prompt_color segment prompt
 
+	# Reversed % marks where a missing trailing newline was; \r\e[K then
+	# clears the line so the prompt always starts cleanly at column 1.
+	printf '%b' '\e[7m%\e[0m\r\e[K'
+
 	local LEFT_PROMPT=""
 	local SEGMENTS_AT_LEFT=0
 	local LAST_SEGMENT_COLOR=""