Server readiness gate: block command intake until initialization complete

[jasonappah]

Objective: Add a readiness/initialization gate so the server does not accept commands until hardware, sockets, and subsystems are ready.

Context:

• src/server/main.ts, src/server/api/socket-manager.ts, and src/server/simulator.ts.
• Optionally expose /healthz or readiness events for the client.

Acceptance:

• During startup, server reports NotReady and rejects commands; once ready, transitions to Ready and accepts.
• Readiness integrates with the motion gating so robots never move before Ready.
• Simulator demonstrates the startup sequence.