Fix group switching with identical colors

[oiahoon]

Summary

• preserve a hidden numeric group identifier after completion-list sorting
• use that identifier for group reload filtering and exact active-header styling
• keep ungrouped reset-colored completion lists unchanged

Testing

• reproduced the failure with identical group-colors before the change
• cd test && zsh -f runtests.zsh fzftab.ztst with the pure Zsh implementation
• zsh -fc 'source ./fzf-tab.zsh && build-fzf-tab-module'
• cd test && zsh -f runtests.zsh fzftab.ztst with the binary module loaded
• zsh -n lib/-ftb-generate-complist lib/ftb-switch-group test/fzftab.ztst

Fixes #581