docs(issue-257): add shared i18n loader acceptance artifact

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

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+# ISSUE-257 Artifact — shared i18n keyspace + frontend loader (Angular-first)
+
+Issue: **#257** (`[MVP][READY] #175-B: Shared i18n keyspace + frontend loader (da/en, Angular-first)`)
+
+## Acceptance mapping
+
+### 1) Delt key-strategi dokumenteret (frontend/backend)
+- Shared contract source: `shared/i18n/lobby.json`
+- Architecture doc: `docs/I18N_ARCHITECTURE.md`
+- Key-map/contract doc: `docs/ISSUE-226-SHARED-KEYMAP-LOCALE-CONTRACT.md`
+
+### 2) Frontend loader kan indlæse da+en med samme keyspace
+- Shared loader: `frontend/shared/i18n/lobby-loader.ts`
+- Angular-first consumer path:
+  - `frontend/src/spa/lobby-i18n.ts`
+  - Angular shell/tests continue to consume same shared catalog through shared loader contract.
+
+### 3) Minimal check for key-paritet da/en
+- Guard test: `frontend/tests/lobby-loader.parity.test.ts`
+- Contract test: `frontend/tests/lobby-i18n.contract.test.ts`
+
+### 4) Ingen API-kontraktbrud
+- Frontend API contract smoke:
+  - `frontend/angular/src/app/api-contract-smoke.spec.ts`
+  - `frontend/tests/angular-api-client.test.ts`
+
+## Verification run (this lane)
+
+```bash
+cd frontend
+npm test -- --run tests/lobby-loader.parity.test.ts tests/lobby-i18n.contract.test.ts tests/angular-api-client.test.ts
+
+cd ../frontend/angular
+npm test -- --run src/app/api-contract-smoke.spec.ts
+```
+
+Result: PASS (all selected suites green).