feat(kernel): console cap gates DebugPutchar, no uart fast path; spawn start arg in a0; delegate returns slot

crates/kernel/src/tests/lifecycle.rs

@@ -11,6 +11,7 @@ fn spawn_thread_creates_ready_thread() {
     let outcome = k.handle_syscall(Syscall::SpawnThread {
         entry: EntryPoint::from_raw(0x1000),
         stack: StackPointer::from_raw(0x2000),
+        arg: 0,
     });
     assert!(matches!(
         outcome,
@@ -19,11 +20,12 @@ fn spawn_thread_creates_ready_thread() {
 }
 
 #[test]
-fn spawn_thread_preserves_entry_and_stack() {
+fn spawn_thread_preserves_entry_stack_and_arg() {
     let (mut k, _) = kernel_with_process();
     let tid = match k.handle_syscall(Syscall::SpawnThread {
         entry: EntryPoint::from_raw(0xDEAD),
         stack: StackPointer::from_raw(0xBEEF),
+        arg: 0x5107,
     }) {
         SyscallOutcome::Complete(SyscallResult::Value(v)) => ThreadId::from_raw(v as u16),
         _ => panic!("expected Value"),
@@ -31,6 +33,11 @@ fn spawn_thread_preserves_entry_and_stack() {
     let ctx = k.thread_context(tid).unwrap();
     assert_eq!(ctx.program_counter(), 0xDEAD);
     assert_eq!(ctx.registers()[2], 0xBEEF);
+    assert_eq!(
+        ctx.registers()[10],
+        0x5107,
+        "the start argument arrives in a0"
+    );
 }
 
 #[test]