FIX: AddrSpace dup handles must NOT be POSITIONED

AddrSpace handles from dup(b'empty') and dup(b'exclusive') use the write
syscall for mmap commands, not seekable I/O. The kwriteoff handler returns
the mapped address (not bytes written), which sys_write would incorrectly
add to the file offset when POSITIONED is set. This corrupted the offset,
causing the next write to fail with EINVAL (offset_word skip exhausted
the buffer). Only mmap-min-addr needs POSITIONED for actual read/write.

src/scheme/proc.rs

@@ -1247,7 +1247,8 @@ impl KernelScheme for ProcScheme {
                         _ => return Err(Error::new(EINVAL)),
                     };
 
-                    handle(Handle { context, kind }, true)
+                    let positioned = !matches!(kind, ContextHandle::AddrSpace { .. });
+                    handle(Handle { context, kind }, positioned)
                 }
                 _ => return Err(Error::new(EINVAL)),
             },