feat(console-web): add bounded iframe keep-alive

- opt service and settings pages into document retention
- preserve iframe identity and local Solid state across navigation
- cap retained documents with deterministic LRU eviction
- deliver activate, deactivate, and dispose lifecycle messages
- reject commands emitted by inactive page documents
- expose page activity as a Solid accessor
- test retention, recreation, eviction, and invalid limits

crates/wasmeld-console/web/src/pages/main.tsx

@@ -46,7 +46,7 @@ function FramePage() {
               </main>
             }
           >
-            <Page state={bridge.state} command={bridge.command} />
+            <Page state={bridge.state} active={bridge.active} command={bridge.command} />
           </Suspense>
         );
       }}

crates/wasmeld-console/web/src/pages/types.ts

@@ -3,5 +3,10 @@ import type { HostSharedState, PageCommand } from "../sdk/protocol";
 
 export type PageProps = {
   state: Accessor<HostSharedState | null>;
+  /**
+   * False while a keep-alive iframe is hidden. Page-owned polling, media or
+   * render loops should pause until this accessor becomes true again.
+   */
+  active: Accessor<boolean>;
   command: (command: PageCommand) => void;
 };