Feat: Implement interconnected organ simulation

This commit refactors the organ simulation to enable dynamic interactions between organs, replacing the previous "faked" or hardcoded connections.

Key changes include:
- Major Refactoring: Changed the `Organ::update` method signature to `update(Patient& patient, double deltaTime_s)`, allowing organs to access the shared patient state and other organs. This was propagated to all organ classes.
- Blood Chemistry Model: Introduced a central `Blood` struct in the `Patient` model to track shared resources like oxygen, CO2, glucose, and toxins.
- Organ System Interconnections:
    - Lungs & Brain: Lungs now perform gas exchange affecting the blood. The brain consumes O2, produces CO2, and its GCS is affected by hypoxia/hypercapnia.
    - Liver-Gallbladder: Gallbladder now receives bile directly from the liver's production rate.
    - Digestive System: Stomach passes chyme to the intestines, which absorb glucose into the blood. The pancreas responds to blood glucose changes.
    - Renal System: Kidneys' GFR is now influenced by the heart's aortic pressure, and they produce urine that fills the bladder directly.
    - Cardiovascular & Neurological: The heart rate responds to hypoxia, and the brain uses live aortic pressure from the heart.
- Comprehensive Test Scenario: Updated the main example to include a 60-second simulation with a meal and a lung injury event to verify the new interconnected system.

This creates a more realistic and scalable physiological simulation framework where organ behaviors are emergent from their interactions.

src/Heart.cpp

@@ -1,4 +1,5 @@
 #include "MedicalLib/Heart.h"
+#include "MedicalLib/Patient.h"
 #include <random>
 #include <algorithm>
 #include <sstream>
@@ -57,11 +58,21 @@ Heart::Heart(int id, int numLeads)
     }
 }
 
-void Heart::update(double deltaTime_s) {
+void Heart::update(Patient& patient, double deltaTime_s) {
     // --- Electrical Simulation Update ---
     totalTime_s += deltaTime_s;
+
+    // Compensatory response to hypoxia
+    double o2_saturation = patient.blood.oxygenSaturation;
+    double targetHeartRate = 75.0;
+    if (o2_saturation < 90.0) {
+        targetHeartRate = 75.0 + (90.0 - o2_saturation) * 2.0; // Increase HR as SpO2 drops
+    }
+
+    // Move current heart rate towards the target
+    heartRate += (targetHeartRate - heartRate) * 0.1 * deltaTime_s;
     heartRate += getFluctuation(0.01); // Slow variation in underlying rate
-    heartRate = std::max(60.0, std::min(heartRate, 100.0));
+    heartRate = std::max(60.0, std::min(heartRate, 140.0));
     double cycleDuration_s = 60.0 / heartRate;
 
     double oldCyclePosition = cardiacCyclePosition_s;