Add English translation of 2-SAT - #402

Add English translation of 2-SAT

Adds content/english/cs/graph-traversals/2-sat.md — an English translation of the Russian 2-SAT article.

How it was produced

Machine-translated with a local model (Seed-X-Instruct-7B, Q8_0, greedy decoding, served by llama.cpp), then hand-reviewed. Math $...$, code spans, the fenced block, link URLs and Markdown structure were protected mechanically before translation and verified byte-identical afterwards (all 66 $...$ spans, all 6 code spans, all 3 URLs, and the cpp block match the source exactly).

Roughly 30 lines were then edited by hand:

• one reversed reachability direction («из $x$ не достижимо $!x$» had come out backwards),
• grammar/calque fixes,
• consistent terminology — «граф импликаций» is rendered as implication graph everywhere,
• typography: curly quotes and em dashes to match the rest of the English pages.

The raw model output is kept separately from the reviewed file, so the amount of human correction is visible.

Things worth your attention

• Scope: only the article itself. No cs/_index.md / graph-traversals/_index.md scaffolding, since content/english/ currently publishes only hpc.
• [We will find](../scc) is a dead link for now — scc exists only in the Russian tree, so it 404s on en.algorithmica.org until the SCC article is translated. Happy to either drop the link or translate scc next.
• prerequisites: - scc also resolves to nothing in the English tree, but header.html uses {{ with $this.GetPage }}, so it is skipped silently.
• The Wikipedia link still points at the Russian article (ru.wikipedia.org/.../Конъюнктивная_нормальная_форма), exactly as in the source. Say the word if you would rather have the English CNF article.
• Front matter: title and weight unchanged, authors transliterated (Sergey Slotin, Maxim Ivanov).
• Hugo is not installed on my machine, so I could not run a local render — relying on the PR preview build.
• The source contains a Cyrillic с inside $с[x]$ on the Rule line (almost certainly a typo in the original); I preserved it rather than silently changing math.