Example: image-driven GUI demo game (announcement artifact)

Description

Extend examples/basic-project or create a new example with an image-driven title screen + HUD: ninepatch panels, labels, interactive buttons wired to game code via typed events.

This the finish line: "ship a game with image-driven GUI worth announcing".

Acceptance criteria

• turian-cli build the example project.
• Run the binary and confirm the same UI renders via sdl3gpu-ontop and buttons dispatch.

Depends on: #99 (closed) (UiDocumentEditor), #101 (closed) (PlayMode input priority), #102 (closed) (sdl3gpu-ontop runtime)