Fix multiplayer join visibility snapshots

[Atchbo2001]

What changed

This is a focused multiplayer-only repair. It intentionally drops the unrelated 20-map package scope from PR #8.

• Sends InitClient / InitAI identity packets before Resume state in the welcome snapshot.
• Sends an immediate visible SpawnState / spectator state with join announcements so existing clients do not learn about a CN without enough state to render it.
• Applies the same visible join snapshot for bots, including bot CNs above 127.

Why

The invis/desync symptom matches a snapshot-order problem: late joiners could receive entity state for a client number before the client identity existed locally, and existing players could receive a join identity without an immediate visible state. That can leave players/bots present in server state but invisible or damage-inconsistent client-side until later traffic happens to repair state.

Scope kept unchanged

No auth changes, no map pack, no browser overlay, no asset replacement, no game-mode removal, no startup/runtime redesign.

Validation

Added focused Go regression tests for:

• late join welcome ordering: identity before resume state
• human join visibility: InitClient followed by SpawnState
• bot join visibility: InitAI followed by SpawnState for CN 128