fix(KeepAliveRoute): preserve children getter to avoid creating sibling routes

[chiefcll]

Summary

Two related fixes inside KeepAliveRoute:

• Stale Show subscribed to a sibling outlet. The component wrapper used a spread to inject isAlive into the route props ({ ...childProps, isAlive }). That spread invokes the router's get children() getter, which builds a <Show when={routeStates()[i+1]} keyed> and subscribes it to routeStates. Because KeepAlive preserves the subtree across navigations, this Show stays alive after the user navigates away. When a later navigation populates routeStates()[i+1] with an unrelated sibling route's context, the stale Show fires and constructs the sibling's component out of the preserved subtree.

  Concrete repro: start at /browse/:filter (a KeepAliveRoute), then navigate to /examples/tmdb. TMDB is invoked twice — once from the stale Show in the preserved Browse subtree, once from the actual Portal render after the lazy import resolves.

  Fix: build the inner props via Object.create(childProps, …) instead of spreading. The children getter stays on the prototype chain and is only invoked when the user component actually reads it. No Proxy, so older platforms (Tizen/WebOS) keep working.

• Route key drift on re-evaluation. <Route> returns a fresh routeDef on every call, and solid-router uses that object identity as the route key (see @solidjs/router createRoutes). If KeepAliveRoute is re-evaluated, the new routeDef changes the key, which forces routeStates to dispose + recreate sibling contexts and re-invoke their components. Cache the resolved <Route> JSX per key so the routeDef identity is stable. The cache captures props at first invocation, so a clearKeepAliveRouteCache() escape hatch is exported for callers that need to refresh.

Test plan

• Verified the repro in the demo app: starting at /browse/all then navigating to /examples/tmdb constructs TMDB exactly once after the fix (previously twice).
• Confirm no regression in the normal mount/unmount/back-navigation cycle for KeepAliveRoute (focus restore, isAlive signal, shouldDispose, preload).
• Manual test: re-entering the keep-alive route preserves state.
• Manual test: forward navigation between two different KeepAliveRoutes.

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