kernel: log unknown syscall numbers before returning ENOSYS

Added println! to the syscall dispatch catch-all to log the
unknown syscall number and arguments when returning ENOSYS.
Previously any unrecognized syscall number silently returned
ENOSYS with no diagnostic, making it impossible to discover
missing syscall implementations without application-level
debugging.

Found by comprehensive disguised-stub audit (47 patterns, 37
actionable). This is the most impactful remaining fix from
that audit.

src/syscall/mod.rs

@@ -276,7 +276,10 @@ pub fn syscall(
             SYS_MPROTECT => mprotect(b, c, MapFlags::from_bits_truncate(d), token).map(|()| 0),
             SYS_MREMAP => mremap(b, c, d, e, f, token),
 
-            _ => Err(Error::new(ENOSYS)),
+            _ => {
+                println!("KERNEL: unimplemented syscall a={:#x} b={:#x} c={:#x} d={:#x}", a, b, c, d);
+                Err(Error::new(ENOSYS))
+            }
         }
     }