Fix #654: Prevent 'Do you wish to see all Possibilities?' prompt

[sbs44]

Description

Fixes #654 - Prevent unnecessary pagination prompt when using colored group descriptions in small terminals

Root Cause

Zsh mistakenly treats strings that are exactly $COLUMNS wide as not fitting on one line, causing the autocomplete system to miscalculate the number of lines needed and trigger the pagination prompt unnecessarily.

Solution

• Centralize display width calculation using _display_width_=$(( COLUMNS - 1)) to avoid code duplication
• Apply consistent width trimming to heading strings to prevent line wrapping
• Preserve the existing logic for parsing and applying heading options (-X/-x)

This ensures that colored group descriptions don't trigger the "Do you wish to see all possibilities?" prompt when they shouldn't.

Testing

Tested with various terminal sizes and with/without colored group descriptions.

Before (unwanted prompt appears):

After (no prompt, completions display correctly):

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