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.

ui/frontend/src/lib/util/ship-class-validation.ts

@@ -6,7 +6,7 @@
 // to gate auto-sync, so the local invariants match the engine's
 // (`game/internal/controller/ship_class.go.ShipClassCreate`).
 //
-// Engine rules (from `pkg/calc/validator.go` and `game/rules.txt`):
+// Engine rules (from `pkg/calc/validator.go` and `site/ru/rules.md`):
 //
 //   - drive, weapons, shields, cargo: float, equal to 0 or >= 1;
 //   - armament: integer, >= 0;