docs(protocol): translate the TikZ figure labels to English

First step of moving the paper from Chinese to English. All 10 figures in
tex/figures.tex now carry English node labels, edge labels and captions, with
node widths and separations re-tuned because English runs noticeably wider
than the Chinese it replaces.

The body text translation and the switch of the LaTeX preamble away from
ctex/CJK follow in the next commit; until then the generated PDF still builds
from the Chinese source, so it will show English figures inside Chinese prose.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q4FAzXMZFjdfqBPY3wSYJ7

Author: claude