Expose simulator world switching to the demo header

[axelpey]

The simulator page's outer header needs to select real worlds without restarting the visitor's session. Add a narrow innate:environment:v1 iframe bridge to the existing SimSession environment command/roster, with source and origin checks, request serialization, connection freshness, and separate physics/model-loading outcomes.

Failed scene downloads now surface a retry. Retrying the same world's geometry preserves the robot and traffic state. Embedded sessions keep their stage during Settings/Navigation detours so the outer selector remains available after the standalone tab's idle cutoff.

Validation:

• Viewer library build and TypeScript check passed; focused environment protocol and existing traffic tests passed.
• Ran the actual local physics/ROS simulator with the companion cloud header: Home → Backrooms → Traffic Lights → Home → Backrooms, checking the loaded scene and authoritative server switch log.
• Exercised a real Nav2 switch failure, injected a missing Traffic Lights model, and verified Retry view restores the scene, robot and traffic without reloading the iframe.
• Checked keyboard selection/Escape, 320px and 390px layouts, a Settings detour beyond 60 seconds, and session-end UI cleanup.

The browser harness serves the real broker frontend with fixture lease/analytics endpoints around a real local simulator. No live broker lease, cloud deployment, or autonomous LLM agent run was tested. Keep this PR draft pending Axel's approval; pair it with innate-cloud #110 before release.