Constant FLOATING_SURFACE_PROTOCOL_VERSION 

pub const FLOATING_SURFACE_PROTOCOL_VERSION: u16 = 25; // 25u16

The app ↔ shell interface’s one version: everything an app and the session shell say on the shell-ui connection (floating-surface lifecycle, the window’s appearance, tabs, selection). The app’s first frame is FloatingSurfaceClientMessage::Hello with this number; a shell at another number answers FloatingSurfaceHostMessage::VersionMismatch and closes, and updates whichever side is behind (docs/architecture/compatibility.md). There is no negotiation and nothing kept for an older number. interface.toml beside this crate holds the schema, and the repository policy fails a change to it that keeps the number.

Geometry is encoded as plain logical-pixel arrays rather than renderer types so sandbox services, Wayland protocol shims, and test transports can all carry the same messages.