docs(#257): add acceptance artifact for shared i18n loader

docs/ISSUE-257-SHARED-I18N-KEYSPACE-FRONTEND-LOADER-ARTIFACT.md

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+# ISSUE-257 Artifact — Shared i18n keyspace + frontend loader (da/en, Angular-first)
+
+Issue: **#257** (`[MVP][READY] #175-B: Shared i18n keyspace + frontend loader`)
+
+## Acceptance checklist
+
+- [x] **Delt key-strategi dokumenteret (frontend/backend)**
+  - Arkitektur/deling beskrevet i `docs/I18N_ARCHITECTURE.md`.
+  - Shared contract + keyspace source of truth: `shared/i18n/lobby.json`.
+
+- [x] **Frontend loader kan indlæse da+en med samme keyspace**
+  - Shared loader: `frontend/shared/i18n/lobby-loader.ts`.
+  - Angular-first integration via `frontend/angular/src/app/lobby-i18n.ts` (samme loader/samme keyspace).
+  - Locale-normalisering inkluderer underscore/hyphen variants (`da_DK` → `da`).
+
+- [x] **Minimal check for key-paritet da/en**
+  - `collectLocaleParityIssues(...)` i shared loader.
+  - Testet i `frontend/tests/lobby-loader.parity.test.ts`.
+
+- [x] **Ingen API-kontraktbrud**
+  - Contract-test: `frontend/tests/lobby-i18n.contract.test.ts`.
+  - Drift-check mellem manifest og katalog: `scripts/check_i18n_drift.py`.
+
+## Kørte checks
+
+```bash
+python3 scripts/check_i18n_drift.py
+cd frontend && npm test -- --run tests/lobby-loader.parity.test.ts tests/lobby-i18n.contract.test.ts
+cd frontend/angular && npm test -- --run src/app/lobby-i18n.spec.ts src/app/i18n-mvp-flow-smoke.spec.ts
+```
+
+Resultat: alle checks grønne.