feat(api): implement gRPC TCP adapter for network access

[rplunger]

Goal

Add a gRPC-over-TCP transport so network clients can connect to the paiOS engine remotely (e.g. from a web dashboard or another machine on the LAN).

Scope

In scope:

• TCP transport for the existing gRPC service definitions
• Feature-gated behind api_grpc_tcp
• Configurable bind address and port
• Reuses the same proto definitions as UDS transport

Out of scope:

• TLS (can layer on later)
• Authentication / API keys
• Rate limiting

Done when

• gRPC server listens on TCP and accepts remote connections
• Same RPCs work over both UDS and TCP
• Feature-gated, compiles with and without
• cargo clippy clean