refactor(#2431): extract hive-priv-sock crate from hive-sh4re

Split the priv-socket wire types (PrivRequest/PrivResponse/PrivEvent and
friends) out of hive-sh4re into their own hive-priv-sock crate, mirroring
the existing hive-host-sock split. hive-priv — the root-privileged
helper — now depends on just this narrow protocol crate instead of the
much larger daemon-shared crate, shrinking its dependency surface and
making the privsep boundary easier to audit. No server/client
implementation lives here, only the wire contract; hive-c0re still
depends on hive-sh4re directly for everything else.

hive-priv/Cargo.toml

@@ -8,7 +8,7 @@ workspace = true
 
 [dependencies]
 anyhow.workspace = true
-hive-sh4re.workspace = true
+hive-priv-sock.workspace = true
 libc.workspace = true
 serde_json.workspace = true
 tokio.workspace = true