docs(spec): Tab strip position — closes #835

Documents the per-theme window.tabPosition setting (top/bottom/left/right)
that closes the 5-year-old feature request and resolves duplicates #9082,
#9100, #10939, #18837. Spec covers design, capabilities, known limitations,
and follow-up work; matches format of existing specs (#532, #4066, #1337,
#1564, #16599).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Sai Sashankh D