Add Godot particle effect pooling

[YfengJ]

Summary

• add a fixed-size ParticlePool autoload for reusable GPUParticles2D instances
• add a ParticleManager autoload that listens to EventBus gameplay signals and dispatches combat, city build, XP, level-up, resource discovery, and flag-capture effects
• add six one-shot particle scenes capped at 200 particles and 1s lifetime for web performance
• add EventBus wrappers for XP gain and resource discovery particle triggers
• add a Node-based Godot particle contract test to guard autoload registration, EventBus wiring, and particle budgets

Closes #46

Validation

• node --test godot/particle_fx_contract.test.mjs
• node --test godot/web/stellar_bridge.test.js
• git diff --check

Not run locally: make godot-export because Godot 4 is not installed in this local environment. The PR keeps the existing Godot Web workflow as the export validator.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud