Same contract as core-v0.3.0 (Device seam, app facade returning serializable
DTOs + DeviceError, pure logic, SVG/PNG card; proven by tests/gui_contract.rs)
with the thiserror 2.0 dependency upgrade folded in.

Consume via: quokka-core = { git = "https://github.com/dutradotdev/quokka.git", tag = "core-v0.3.1" }