docs(site): edit rules for clarity + cross-links; migrate off rules.txt

Editorial pass over site/ru/rules.md (on top of the verbatim port):
- moved the lore intro to the RU home page, rewritten in a modern voice;
- fixed typos, replaced the TODO/WTF cargo-tech note and the abandoned
  (---ссылка---) marker with the verified mechanic and a real cross-link,
  dropped the report TODO row;
- wove organic intra-page cross-links (#combat, #movement, #victory, ...);
- documented engine nuances verified against the code: ore auto-farming
  and the capital / "запасы промышленности" store (industry capped at
  population); cargo lost with ships destroyed in battle; and that a
  losing race's colonists at a neutral planet are NOT lost — they stay
  aboard (this corrects the audit note, verified in route.go).

Migration: delete game/rules.txt (its content now lives, authoritative,
in site/ru/rules.md) and repoint every reference to it (ui/frontend code
comments + tests, ui/docs, tools, ui/PLAN.md links). Record the
RU-authoritative rule in site/README.md and CLAUDE.md. The English
site/rules.md mirror follows in a separate stage.

site/README.md

@@ -20,6 +20,11 @@ under `/game/` (see `tools/dev-deploy/Caddyfile.dev`).
 - Add a page as Markdown and register it in the `sidebar` of each locale
   in `.vitepress/config.ts`.
 - Localised content mirrors the English tree under `ru/`.
+- **Game rules** (`ru/rules.md`) are the one exception to English-first:
+  the Russian page is the authoritative source (ported from the former
+  `game/rules.txt`), and the English `rules.md` is its mirror/translation.
+  Keep the two in sync as with `FUNCTIONAL.md`/`FUNCTIONAL_ru.md`, but with
+  Russian leading.
 - Math uses LaTeX: inline `$E = mc^2$`, block `$$ … $$`.
 - Link to the game with the root-relative `/game/` path so the build
   stays domain-agnostic (no hard-coded host).