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/tests/science-validation.test.ts

@@ -112,7 +112,7 @@ describe("validateScience", () => {
 		expect(result.reason).toBe("sum_not_hundred");
 	});
 
-	test("accepts the canonical First Step fixture from rules.txt", () => {
+	test("accepts the canonical First Step fixture from site/ru/rules.md", () => {
 		// 10 Drive + 5 Weapons + 30 Shields + 0 Cargo, normalised:
 		// 10/45 ≈ 22.222… %, 5/45 ≈ 11.111… %, 30/45 ≈ 66.666… %,
 		// 0/45 = 0 %. Snapped to one decimal at input time: