feat(session-server): add session server for app launching, remove apps from init manifest

Init now only starts servers. The session server holds ProcessCreate cap
and launches apps on demand. Snake is no longer in the boot manifest.

crates/servers/session-server/src/protocol.rs

@@ -0,0 +1,77 @@
+use server_framework::{Decode, Encode};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct AppId(u16);
+
+impl AppId {
+    pub const fn new(id: u16) -> Self { Self(id) }
+    pub const fn raw(&self) -> u16 { self.0 }
+}
+
+#[derive(Debug, PartialEq)]
+pub enum SessionRequest {
+    Launch { app: AppId },
+    Kill { app: AppId },
+    ListApps,
+    ActiveApp,
+}
+
+#[derive(Debug, PartialEq)]
+pub enum SessionResponse {
+    Launched { app: AppId },
+    Killed,
+    AppList { count: usize },
+    Active { app: AppId },
+    NotFound,
+    Error,
+}
+
+impl Encode for SessionRequest {
+    fn encode(&self) -> [usize; 4] {
+        match self {
+            SessionRequest::Launch { app } => [0, app.raw() as usize, 0, 0],
+            SessionRequest::Kill { app } => [1, app.raw() as usize, 0, 0],
+            SessionRequest::ListApps => [2, 0, 0, 0],
+            SessionRequest::ActiveApp => [3, 0, 0, 0],
+        }
+    }
+}
+
+impl Decode for SessionRequest {
+    fn decode(words: [usize; 4]) -> Option<Self> {
+        match words[0] {
+            0 => Some(SessionRequest::Launch { app: AppId::new(words[1] as u16) }),
+            1 => Some(SessionRequest::Kill { app: AppId::new(words[1] as u16) }),
+            2 => Some(SessionRequest::ListApps),
+            3 => Some(SessionRequest::ActiveApp),
+            _ => None,
+        }
+    }
+}
+
+impl Encode for SessionResponse {
+    fn encode(&self) -> [usize; 4] {
+        match self {
+            SessionResponse::Launched { app } => [0, app.raw() as usize, 0, 0],
+            SessionResponse::Killed => [1, 0, 0, 0],
+            SessionResponse::AppList { count } => [2, *count, 0, 0],
+            SessionResponse::Active { app } => [3, app.raw() as usize, 0, 0],
+            SessionResponse::NotFound => [4, 0, 0, 0],
+            SessionResponse::Error => [5, 0, 0, 0],
+        }
+    }
+}
+
+impl Decode for SessionResponse {
+    fn decode(words: [usize; 4]) -> Option<Self> {
+        match words[0] {
+            0 => Some(SessionResponse::Launched { app: AppId::new(words[1] as u16) }),
+            1 => Some(SessionResponse::Killed),
+            2 => Some(SessionResponse::AppList { count: words[1] }),
+            3 => Some(SessionResponse::Active { app: AppId::new(words[1] as u16) }),
+            4 => Some(SessionResponse::NotFound),
+            5 => Some(SessionResponse::Error),
+            _ => None,
+        }
+    }
+}