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-sock/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "hive-priv-sock"
+edition.workspace = true
+version.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+serde.workspace = true