feat(ipc): saved replies: SaveReply/ReplySaved (8 per thread, released on thread death, unforgeable); terminal READ saved until Enter (line.rs editor, reader queue), now pinged

crates/domain/src/syscall.rs

@@ -1,7 +1,7 @@
 use types::{
     address::{DataRef, EntryPoint, StackPointer, VirtualAddress},
     badge::Badge,
-    bounded::{CapSlot, PageCount},
+    bounded::{CapSlot, PageCount, SavedReply},
     mapping::PagePerms,
     permission::PermissionSet,
     time::Ticks,
@@ -104,6 +104,14 @@ pub enum Syscall {
         notification: CapSlot,
         deadline: Ticks,
     },
+    /// Keeps the caller this thread owes a reply to, to answer later
+    /// (`ReplySaved`) while it receives others; returns which it is.
+    SaveReply,
+    /// Answers a caller kept with `SaveReply`, once.
+    ReplySaved {
+        reply: SavedReply,
+        message: Message,
+    },
     /// Removes a capability from the caller's own table.
     DropCap {
         slot: CapSlot,