Add platform-aware IPC transport layer for the launcher protocol:
- transport module: send_request_sync() auto-selects doors (illumos)
or Unix sockets (Linux) at compile time
- doors_transport: uses doors::Client for high-speed synchronous RPC,
gated behind cfg(target_os = "illumos") + "doors" feature flag
- unix_transport: blocking Unix socket client for all platforms
Simplify client binaries to use the sync transport:
- wradm: drops tokio dependency entirely, now fully synchronous
- wrlogin: drops tokio dependency, uses sync transport
- wrsessd: uses shared default_ipc_path() for consistency
The doors crate (0.8.1) is an optional illumos-only dependency.
On Linux, all code compiles and tests pass with Unix socket transport.