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

[seefood]

Summary

When a command produces output without a trailing newline (e.g. echo -n foo), the existing \e[G (move to column 1) in PS1 overwrites that output with the prompt. This adds a missing-newline handler at the top of __powerline_prompt_command:

printf '%b' '\e[7m%\e[0m\r\e[K'

How it works:

• \e[7m%\e[0m — prints a reversed-video % at the current cursor position. If the previous command left the cursor mid-line, this character is briefly visible as a "missing newline" indicator (same convention as zsh's PROMPT_SP)
• \r — carriage return to column 1 of the current line
• \e[K — clear from column 1 to end of line

Result:

• Previous command ended with newline: cursor was already at column 1 on a blank line → % is printed and immediately cleared → no visible change
• Previous command had no trailing newline: % appears at the end of the partial output, then \r\e[K clears the entire line → prompt renders cleanly on what is now an empty line

The existing \e[G in PS1 is kept as-is (it becomes a harmless no-op since we are already at column 1 after the printf).

Note: powerline-multiline uses \e[B (cursor down) instead of \e[G and has different behaviour; that theme is out of scope for this fix.

Test plan

• echo -n foo — verify % appears briefly then prompt renders on a clean new line, foo is not overwritten
• Normal commands (echo foo, ls) — verify no extra blank line is inserted
• Works in powerline, powerline-plain, and powerline-naked (plain and naked inherit __powerline_prompt_command from base)

Fixes #2372

🤖 Generated with Claude Code

[seefood]

Updated the implementation based on the feedback in #2372: the reversed-% + \e[K approach cleared the previous command's unterminated output instead of preserving it (as joshiste reported).

Replaced it with the approach joshiste suggested: overflow the current line with COLUMNS-1 spaces so the terminal auto-wraps the cursor onto a fresh line, then \r to column 1. This preserves any partial output from the previous command instead of erasing it. beginning_of_line (\e[G) is now redundant since the cursor is already at column 1 by the time PS1 is rendered, so it was removed.

Tests updated accordingly (test/themes/powerline.base.bats), including a fallback check for when COLUMNS is unset.