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

crates/libkos/src/io.rs

@@ -4,8 +4,11 @@ struct Writer;
 
 impl fmt::Write for Writer {
     fn write_str(&mut self, s: &str) -> fmt::Result {
-        for &b in s.as_bytes() {
-            crate::syscall::debug_putchar(b);
+        // Without a console capability, output goes nowhere.
+        if let Some(console) = crate::start::console() {
+            for &b in s.as_bytes() {
+                crate::syscall::debug_putchar(console, b);
+            }
         }
         Ok(())
     }