test(ensemble): cover navigateViewGroup clamp and CDN stale refresh paths

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Risky behavior now covered

• navigateViewGroup index resolution (0ce6723a): When a script passes an out-of-range viewIndex, the action must clamp before PageController.jumpToPage to avoid assertion failures. Tests cover both the clamp path (known menu length) and the passthrough path (menu length unknown).
• CDN stale-refresh race condition (19b50b2e): After a background CDN fetch updates the manifest, the provider must either apply changes immediately (artifact refresh enabled + app initialized) or defer to resume. Tests lock in both branches so the cold-start vs deferred behavior cannot regress.

Test files added/updated

• modules/ensemble/test/safe_view_group_payload_index_test.dart — resolveNavigateViewGroupTabIndex cases
• modules/ensemble/test/cdn_provider_test.dart — CDN stale refresh immediate vs deferred outcomes

Why these tests materially reduce regression risk

• Navigation: jumpToPage crashes are user-facing and hard to catch without integration tests; the extracted helper mirrors production logic exactly.
• CDN updates: The race fix depends on ordering (bundle update before refresh event) and on choosing the correct apply/defer branch; unit tests assert bundle version and pending-flag state without network or lifecycle flakiness.

Validation

flutter test test/safe_view_group_payload_index_test.dart test/cdn_provider_test.dart

All 14 tests in those files pass.