fix(debug): report EVENT_READ in fevent when serial input is queued

The debug scheme's fevent always returned empty, relying solely on the
edge-triggered debug_notify(). A reader that registers with an event queue after
input has already arrived (fbcond's serial console) would miss it. Report
readable when the input queue is non-empty.

src/scheme/debug.rs

@@ -109,7 +109,19 @@ impl KernelScheme for DebugScheme {
         _flags: EventFlags,
         token: &mut CleanLockToken,
     ) -> Result<EventFlags> {
-        let _handle = *HANDLES.read(token.token()).get(id)?;
+        let handle = *HANDLES.read(token.token()).get(id)?;
+
+        // Report readable when serial/debug input is already queued, so a reader
+        // that registers with an event queue AFTER input has arrived (e.g.
+        // fbcond's serial console feeding the getty) still learns it is
+        // readable, rather than relying solely on the edge-triggered
+        // debug_notify(). Non-input handles never report readable.
+        if handle.num != SpecialFds::DisableGraphicalDebug as usize
+            && handle.num != SpecialFds::SchemeRoot as usize
+            && !INPUT.is_currently_empty(token)
+        {
+            return Ok(EVENT_READ);
+        }
 
         Ok(EventFlags::empty())
     }