docs: language model + liturgical content policies, fix ru/services drift

- .ai-specs/PROJECT_CONTEXT.md: new 'Language Model (UI vs. Content)' section
  (serviceLang ru->cu, fallback chain, scripture, Bible tab independence,
  fonts, calendar authority) + content-policy pointer in Critical Invariants.
- .ai-specs/WORKING_POLICIES.md: new 'Liturgical Content Policies' —
  never translate service/Bible texts (sourced only), cu texts verbatim
  (no normalisation/latinisation), service assembly follows the Typicon.
- DATA_EN.md/DATA_RUS.md: ru/ holds enriched lives only; services fall back
  to cu; cu is the authoritative calendar + service texts.

DATA_EN.md

@@ -51,8 +51,8 @@ static/data/
 │   ├── menaion/                     # Daily menaion entries (366 days)
 │   ├── calendar/                    # Calendar metadata
 │   └── services/                    # Service templates
-├── cu/                              # Church Slavonic (similar structure)
-├── ru/                              # Russian (lives, services)
+├── cu/                              # Church Slavonic (authoritative calendar + service texts)
+├── ru/                              # Russian (enriched lives; services fall back to cu)
 ├── el/                              # Greek (includes mono/)
 ├── fr/                              # French
 ├── zh/                              # Chinese (includes Hans/, Hant/)