Add activation gates and sessions; wire runtime services

[LegendaryForge]

Summary

• Add activation gates and activation sessions as runtime services.

Changes

• Add Gate API (api/gate) and default implementation.
• Add Activation + ActivationSession APIs (api/activation, api/activation/session).
• Add default internal services for activation/session.
• Wire GateService, ActivationSessionService, ActivationService into DefaultCoreRuntime.

Rationale

• Enables content mods to enforce activation authority, condition gates, and activation sessions without Core policy bloat.
• Provides minimal Core seams required for player-visible activation flows (Thunderfury as stress test).

Verification

• ./gradlew spotlessApply clean test