Add 2.5D canvas-based interactive world demo

[PGuuu]

Motivation

• Provide a small, self-contained 2.5D interactive world demo inspired by early adventure games to explore movement, simple interactions, and sprite ordering.

Description

• Add index.html which hosts the canvas and HUD for messages and loads the game script.
• Add style.css with layout, canvas styling, and HUD/message visuals.
• Add game.js which implements the tile map, obstacles/interactables, NPC, player state, smooth grid-based movement (tryMove), interaction logic (interact), and a render loop that sorts sprites by y to simulate depth.
• The map includes water/cliff/path/grass tiles, trees, a house, a sign, and sample interactable messages to demonstrate gameplay flow.

Testing

• Started a local server with python -m http.server 8000 and successfully ran a Playwright script that loaded http://127.0.0.1:8000/index.html and produced a screenshot at artifacts/particlesystem.png.
• No unit tests were added for this demo; manual/visual verification was performed via the automated browser script which completed successfully.

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Codex Task