wip: subcontainer exec log drain via SCM_RIGHTS (reference only)

Implemented pipe FD handoff from exec to launch via Unix socket +
SCM_RIGHTS for grandchild log capture. Superseded by the simpler
PR_SET_DUMPABLE approach which eliminates the need for pipes entirely.

core/Cargo.toml

@@ -161,6 +161,7 @@ nix = { version = "0.30.1", features = [
   "process",
   "sched",
   "signal",
+  "socket",
   "user",
 ] }
 nom = "8.0.0"