feat: Implement major physiological feedback loops

This commit re-implements several critical physiological feedback loops that were lost, enhancing the realism of the simulation.

The following systems have been added:

1.  **Full Digestive Loop:**
    - The Gallbladder now has a `releaseBile` method, triggered by chyme in the intestines.
    - The Pancreas has a `releaseEnzymes` method, also triggered by chyme.
    - The Intestines' digestion logic has been updated to be more effective when bile and enzymes are present.

2.  **Autonomic Nervous System Control:**
    - The Brain now monitors blood gas (O2/CO2) and blood pressure levels.
    - It dynamically adjusts the respiration rate of the Lungs via a new `setRespirationRate` method in response to blood gas changes.
    - It controls the heart rate via a new `setHeartRate` method in response to blood pressure changes, simulating the baroreceptor reflex.
    - The previous hardcoded rate control logic in the Lungs and Heart has been removed.

3.  **Kidney Blood Pressure Regulation (RAAS):**
    - A simplified Renin-Angiotensin-Aldosterone System has been implemented.
    - The Kidneys now secrete renin in response to low blood pressure.
    - The Liver produces a constant supply of angiotensinogen.
    - A new `angiotensin_au` value in the Blood struct is calculated in the main patient update loop.
    - This hormone now acts as a vasoconstrictor, directly affecting the blood pressure calculation in the Heart.

These changes significantly increase the complexity and fidelity of the medical simulation by modeling the interconnectedness of the major organ systems.

src/Kidneys.cpp

@@ -18,6 +18,7 @@ static double getFluctuation(double stddev) {
 
 Kidneys::Kidneys(int id)
     : Organ(id, "Kidneys"),
+      reninSecretionRate(1.0), // Baseline ng/mL/hr
       gfr_mL_per_min(125.0),
       urineOutput_mL_per_s(0.02),
       bloodSodium_mEq_per_L(140.0),
@@ -64,11 +65,24 @@ void Kidneys::update(Patient& patient, double deltaTime_s) {
     bloodSodium_mEq_per_L += getFluctuation(0.05);
     bloodPotassium_mEq_per_L += getFluctuation(0.01);
 
+    // --- RAAS Regulation ---
+    // Renin is released in response to low blood pressure.
+    const auto& bp = patient.blood.bloodPressure;
+    double meanArterialPressure = bp.diastolic_mmHg + (bp.systolic_mmHg - bp.diastolic_mmHg) / 3.0;
+
+    if (meanArterialPressure < 85.0) { // Low pressure threshold
+        reninSecretionRate += (85.0 - meanArterialPressure) * 0.1 * deltaTime_s;
+    } else {
+        // If pressure is normal, renin decays back to baseline
+        reninSecretionRate -= (reninSecretionRate - 1.0) * 0.05 * deltaTime_s;
+    }
+
     // Clamp to healthy ranges
     gfr_mL_per_min = std::clamp(gfr_mL_per_min, 90.0, 150.0);
     urineOutput_mL_per_s = std::clamp(urineOutput_mL_per_s, 0.01, 0.03);
     bloodSodium_mEq_per_L = std::clamp(bloodSodium_mEq_per_L, 135.0, 145.0);
     bloodPotassium_mEq_per_L = std::clamp(bloodPotassium_mEq_per_L, 3.5, 5.0);
+    reninSecretionRate = std::clamp(reninSecretionRate, 0.5, 50.0);
 }
 
 std::string Kidneys::getSummary() const {
@@ -78,6 +92,7 @@ std::string Kidneys::getSummary() const {
     ss << "--- Kidneys Summary ---\n"
        << "Glomerular Filtration Rate (GFR): " << getGfr() << " mL/min\n"
        << "Urine Output: " << getUrineOutputRate() * 3600 << " mL/hr\n"
+       << "Renin Secretion: " << getReninSecretionRate() << " ng/mL/hr\n"
        << "Blood Sodium: " << getBloodSodium() << " mEq/L\n"
        << "Blood Potassium: " << getBloodPotassium() << " mEq/L\n";
     return ss.str();
@@ -88,3 +103,4 @@ double Kidneys::getGfr() const { return gfr_mL_per_min; }
 double Kidneys::getUrineOutputRate() const { return urineOutput_mL_per_s; }
 double Kidneys::getBloodSodium() const { return bloodSodium_mEq_per_L; }
 double Kidneys::getBloodPotassium() const { return bloodPotassium_mEq_per_L; }
+double Kidneys::getReninSecretionRate() const { return reninSecretionRate; }