refactor(#2862): drop the fd framing module, the hazard is unreachable

The Framer bound a passed descriptor to the request line it belongs to,
on the premise that several requests can be in flight on one connection
so a descriptor could arrive with a chunk belonging to a different one.

That premise is false. hive-sock-client::try_once connects per request
(connect, write one line, read one line, drop) and priv_client's two
connect sites each open their own stream, so a connection carries
exactly one request: one line, at most one descriptor, nothing to
disambiguate. Request and response align by connection.

Delete it rather than move it. The recvmsg swap still has to happen —
SCM_RIGHTS is attached to a specific recvmsg call and BufReader::lines
cannot surface it — but the pairing it needs is "take the descriptor
that arrived with this line", not a queue and a claim policy.

hive-priv-sock/src/lib.rs

@@ -12,12 +12,6 @@
 
 use serde::{Deserialize, Serialize};
 
-// Line framing that also carries passed file descriptors. Part of the
-// wire contract rather than one side's implementation detail: the
-// daemon sends descriptors and the helper reassembles them, so both
-// ends have to agree on how a descriptor is bound to a request.
-pub mod framing;
-
 /// Default socket path for the privileged helper.
 pub const PRIV_SOCK: &str = "/run/hive/priv.sock";