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

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Risky behavior now covered

• navigateViewGroup index clamping (0ce6723a): resolveNavigateViewGroupTabIndex locks in the rule that out-of-range indices are clamped before PageController.jumpToPage when the menu has items, while preserving the raw index when menuLen == 0 (PageGroup not mounted).
• CDN stale refresh race (19b50b2e): _applyStaleRefreshOutcome / shouldApplyCdnStaleRefreshImmediately cover the cold-start path where artifact refresh is enabled and the app is initialized—bundle sync happens immediately instead of deferring to resume, preventing screens from rebuilding with stale resources.

Test files added/updated

• modules/ensemble/test/safe_view_group_payload_index_test.dart — resolveNavigateViewGroupTabIndex clamp and zero-menu edge cases
• modules/ensemble/test/cdn_provider_test.dart — stale refresh decision matrix and immediate vs deferred bundle sync

Why these tests materially reduce regression risk

• Navigation: A regression that stops clamping viewIndex before jumpToPage would crash or land on the wrong tab when payloads shrink; the extracted helper makes that contract explicit and unit-testable without a full widget tree.
• CDN updates: The race fix depends on applying bundle changes before firing refresh events. Tests assert immediate sync when artifact refresh + config are present, and that updates are deferred (pending flag set, bundle unchanged) when refresh is disabled—guarding against reintroducing the stale-resource rebuild race.

Validation

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

All 15 tests passed.