Debug: Spawn players for 4-player start

[Ridderrasmus]

Add a debug-only way to spawn 3 additional players so a game can start with 4 players during QA.

Acceptance:

• Debug path to add 3 players (no manual multi-tab required)
• Works in dev/QA only; not in production
• Update QA runbook if needed

[github-actions]

📋 Assigned to Ripley (Lead)

Issue: #57 — Debug: Spawn players for 4-player start

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Ripley, work on issue #57