fix(observatory): synchronize scenario caption

[alloutflo]

Summary

• derive the Observatory caption from demoData.currentScenario on every HUD frame
• repair stale startup text even when the internal scenario key already matches
• avoid redundant DOM writes and keep edge-module rebuilding change-only
• add the initialization regression to the blocking PR UI job

Validation

• node --check ui/observatory/js/hud-controller.js
• PR UI command — 27/27 passed
• git diff --check upstream/main...HEAD

This covers the HUD/DOM boundary; no browser animation or hardware run is claimed.

Closes #1527