refactor: replace socket magic 0x534F434B with proper VFS FS_SOCKET nodes

refactor: replace socket magic 0x534F434B with proper VFS FS_SOCKET nodes

- Add FS_SOCKET type to fs.h
- Create sock_node_create/close/read/write: proper fs_node_t for sockets
  with read→ksocket_recv, write→ksocket_send, close→ksocket_close
- Socket ID stored in node->inode (previously in file->offset)
- sock_fd_get_sid helper validates socket FDs via FS_SOCKET type check
- socket()/accept() now create VFS nodes instead of magic-flagged files
- fd_close no longer needs special socket magic check
- read()/write() on socket FDs now work via standard VFS dispatch
- All 0x534F434BU magic references eliminated from codebase