test: add config validation and viewer integration tests

- Config validation suite (43 tests): verify HLS grid config, manifest
  dedup, video entry normalization, storage pipeline metadata preservation
- Viewer integration tests (30 tests): panel rendering for HLS/direct
  entries, edge cases (empty manifests, missing fields, mixed grids)
- Tests mirror current pipeline functions (not outdated code)

__tests__/run.mjs

@@ -5,6 +5,7 @@ import './schema.test.mjs';
 import './viewer_units.test.mjs';
 import './data_flow.test.mjs';
 import './viewer_integration.test.mjs';
+import './config_validation.test.mjs';
 import { run } from './runner.mjs';
 
 const result = await run();