Fix task validation: accept OpenEnv action dict format in /step, add /grade endpoint, run all 3 tasks in inference.py

Fix Phase 2 Task Validation: add missing OpenEnv required API endpoints

server fixes

server fixes

minior fixes

Fix FileNotFoundError for interactive_demo.html

Minor fixes

Add HTML landing page for HF Space App tab

Add root / endpoint to fix HF Space 404

Research improvements: physics fidelity + new server endpoints

Rebuild as structural_design_env: replace pgsa_env with full OpenEnv implementation