Add minimal Godot export surface and restore build by implementing RespiratorySystem

[Copilot]

The simulation could not be used for Godot visualization because Body referenced a non-existent RespiratorySystem, and there was no stable payload for external rendering. This change adds the missing respiratory system and a JSON snapshot API designed for Godot consumption.

• Body initialization + system completeness

  • Added RespiratorySystem so Body can construct all currently referenced systems.
  • Implemented minimal respiratory nodes/components and handling for SuffocateEvent / ClearAirwayEvent.

• Godot-facing state export

  • Added Body.ExportForGodotJson(bool indented = false).
  • Export includes:
    • global resource pool (resources)
    • per-system body-part state (systems)
    • node status + component metrics (current, max, regenRate)

• Serialization access point in system base

  • Added BodySystemBase.GetNodes() to expose node state as a read-only map for export without changing existing system behavior.

• Focused coverage for new contract

  • Added BodyTests validating:
    • respiratory system is initialized in Body
    • exported JSON includes expected systems/resources/component fields for visualization clients

var body = new Body();
string snapshot = body.ExportForGodotJson(indented: true);
// snapshot["systems"]["Respiratory"]["Chest"]["components"]["AirFlow"]["current"]

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.