Implement random special events (Space Hag, Gold Transport, derelicts, mysterious trader, pirate traps)

[rdearman]

Add canonical random encounters and events.

Parent: #437

Summary:

• Implement Space Hag, Gold Transport, derelict ship, mysterious trader, pirate trap and similar events.
• Trigger them on movement or time-based schedules.
• Provide appropriate rewards/risks.

Acceptance criteria:

• Event generation framework with tunable probabilities.
• Events reported via in-game messages and news.
• No severe performance impact.

See epic for canon doc links.