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/adapter-riscv/src/platform.rs

@@ -126,4 +126,17 @@ impl Platform for RiscVPlatform {
             ),
         });
     }
+
+    fn halt(&self) -> ! {
+        unsafe {
+            core::arch::asm!(
+                "ecall",
+                in("a0") 0usize,
+                in("a1") 0usize,
+                in("a7") 0x53525354usize,
+                in("a6") 0usize,
+            );
+        }
+        loop { unsafe { core::arch::asm!("wfi") }; }
+    }
 }