feat(gateway): startup discovery summary + empty-graph diagnostics + self-entity

[mfaferek93]

The gateway boots cleanly and serves an empty entity tree when it cannot see the target ROS graph (wrong ROS_DOMAIN_ID, mismatched RMW_IMPLEMENTATION, container network isolation, ROS_LOCALHOST_ONLY, blocked multicast). Today there is no startup summary, no empty-graph warning, no environment echo, and no gateway self-entity (gateway_node.cpp logs its config but never the discovered entity count). The result is a silent dead end: the user sees a blank tree with no hint where to look.

Scope

• Startup discovery summary: after the first discovery cycle, log the number of discovered nodes / topics / medkit entities, the gateway URL, and a sample curl.
• Empty-graph diagnostics, surfaced to the USER (not console-only): when zero application entities are discovered, report the likely causes and echo the active environment (ROS_DOMAIN_ID, RMW_IMPLEMENTATION, ROS_LOCALHOST_ONLY) somewhere the UI/API can show it (for example on /health or as a gateway self-entity attribute), so a user staring at an empty tree sees "0 application nodes visible - domain X, rmw Y, localhost-only Z - check that the gateway shares the robot's domain / RMW / network" and knows where to look. Also log it loudly on the console.
• Gateway self-entity: the gateway always registers itself as an entity, so the tree is never literally blank and the "0 application entities" state is explicit rather than empty.
• Document the co-location requirement and multi-domain topology in the bringup quickstart (feat: single bringup launch for gateway + fault_manager + bridges #424): the gateway must share ROS_DOMAIN_ID + RMW_IMPLEMENTATION + network with the target stack; multiple domains run one gateway per domain federated by peer aggregation.

Acceptance

• On startup the gateway logs discovered node / topic / entity counts plus the URL and a sample curl.
• With no reachable application nodes, the gateway both logs a warning and exposes the same diagnostic via the UI/API (not console-only), including the active ROS_DOMAIN_ID / RMW_IMPLEMENTATION / ROS_LOCALHOST_ONLY and the likely causes.
• The gateway self-entity is always present; the entity tree is never empty even before any application node is discovered.