feat: add Halt syscall, fault notification to fault handler, supervisor with back-off, shutdown with timeout

- Halt syscall: only pid 0 (init) can halt, adapter uses SBI SRST
- Fault notification: kernel delivers fault info to process fault handler endpoint before killing
- Supervisor: tracks restart counts, back-off up to 5 restarts
- Shutdown: notifies servers, waits with timeout, calls halt
- Trap handler: faults now call kernel.handle_fault instead of halting

crates/domain/src/fault.rs

@@ -8,3 +8,23 @@ pub enum Fault {
     MemoryFault { address: VirtualAddress },
     IllegalInstruction,
 }
+
+impl Fault {
+    pub fn code(&self) -> usize {
+        match self {
+            Fault::CapabilityViolation { .. } => 0,
+            Fault::InvalidCapability { .. } => 1,
+            Fault::RevokedCapability { .. } => 2,
+            Fault::InvalidSyscall => 3,
+            Fault::MemoryFault { .. } => 4,
+            Fault::IllegalInstruction => 5,
+        }
+    }
+
+    pub fn address(&self) -> usize {
+        match self {
+            Fault::MemoryFault { address } => address.raw(),
+            _ => 0,
+        }
+    }
+}

crates/domain/src/syscall.rs

@@ -86,4 +86,5 @@ pub enum Syscall {
     GetProcessId,
     Yield,
     Exit,
+    Halt,
 }