usb: build the full KVM composite surface

Cargo.toml

@@ -73,6 +73,13 @@ ratatui-ratty = { version = "0.2", optional = true }  # Inline 3D graphics via R
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 
+# Full USB KVM receiver. Optional because only the aarch64 phone package ships
+# it; the main substrate and desktop builds do not need a windowing stack.
+gadgetry-most-foul = { version = "0.1.1", optional = true }
+gud-gadget = { git = "https://github.com/samcday/gud-gadget", rev = "33024b6152633eb8359375419281fa3ebac9c9cb", optional = true }
+softbuffer = { version = "0.4", optional = true }
+winit = { version = "0.30", optional = true }
+
 # Error handling
 anyhow = "1"
 thiserror = "1"
@@ -215,6 +222,11 @@ required-features = ["sessiond"]
 name = "souveraine-sensord"
 path = "src/bin/souveraine-sensord.rs"
 
+[[bin]]
+name = "souveraine-usb-kvm"
+path = "src/bin/souveraine-usb-kvm.rs"
+required-features = ["usb-kvm"]
+
 [profile.release]
 opt-level = 3
 lto = true
@@ -238,10 +250,14 @@ gui = ["tuie/gui"]
 secrets = ["dep:zbus", "dep:zvariant", "dep:aes", "dep:cbc", "dep:cipher", "dep:hkdf", "dep:num-bigint", "dep:num-traits", "dep:aes-gcm", "dep:argon2"]
 # Session authority daemon — holds ext-session-lock before/under the shell.
 sessiond = ["dep:wayland-client", "dep:wayland-protocols"]
+usb-kvm = ["dep:gadgetry-most-foul", "dep:gud-gadget", "dep:softbuffer", "dep:winit"]
 wry = ["dep:wry"]
 tao = ["dep:tao"]
 gtk = ["dep:gtk"]
 
+[patch."https://github.com/samcday/gadgetry-most-foul.git"]
+gadgetry-most-foul = { version = "0.1.1" }
+
 [[bin]]
 name = "souveraine-web"
 path = "src/bin/souveraine-web.rs"