feat(ffi): add bloodstream and bladder metrics API

- add C FFI enums MLBladderPhase, MLMetabolicState, MLPerfusionState
- add C FFI structs MLBloodstreamMetrics and MLBladderMetrics
- add ml_patient_bloodstream_metrics and ml_patient_bladder_metrics
  to populate metrics for a patient (mirrored in src/ffi.rs)
- update examples/c/ffi_example.c to print new metrics
- add tests for FFI metrics (tests/ffi.rs)

organ model expansions
- bloodstream: add metrics() snapshot and detailed physiology:
  plasma proteins/oncotic pressure, lymph return, RBC cohort tracking,
  erythropoiesis/clearance with HIF, iron/folate/B12 stores, platelets,
  coagulation/fibrinolysis, immune cell counts, complement, acid–base
- bladder: introduce adaptive compliance, reflex gating, cortical/voluntary
  modulators, safety indices; add metrics(), summary, and unit tests
- brain: add homeostatic drives (respiratory, thirst, hunger, thermo, pain),
  brainstem nuclei (NTS/RVLM/CVLM, nAmb/DMV, RTN), sleep cycle timing,
  cerebrovascular autoregulation; wire drives into autonomic control
- heart: add phase-based cycle (valves and atria), conduction system,
  RAAS regulation, improved coronary perfusion
- intestines: add micronutrient absorption feeding erythropoiesis

patient coupling
- expose Patient::bloodstream_metrics() and ::bladder_metrics()
- integrate new organ signals (kidney osmolality, spleen culling, liver
  proteins) and brain–lung/continence control pathways
- re-export BladderMetrics and BloodstreamMetrics in lib.rs

note
- existing FFI remains compatible; this is a surface addition
- ffi/medicallib.h kept in sync with src/ffi.rs

tests/bladder_metrics.rs

@@ -0,0 +1,70 @@
+use medicallib_rust::{Bladder, Organ, OrganType, Patient};
+
+#[test]
+fn bladder_metrics_match_patient_api() {
+    let patient = Patient::new("metrics")
+        .unwrap()
+        .initialize_default()
+        .with_organ(OrganType::Bladder);
+    let metrics_via_patient = patient.bladder_metrics().expect("bladder present");
+    let bladder = patient.find_organ_typed::<Bladder>().unwrap();
+    let direct = bladder.metrics();
+    assert!((metrics_via_patient.urgency - direct.urgency).abs() < 1e-6);
+    assert_eq!(metrics_via_patient.phase, direct.phase);
+    assert_eq!(metrics_via_patient.capacity_ml, direct.capacity_ml);
+}
+
+#[test]
+fn voluntary_hold_updates_metrics() {
+    let mut patient = Patient::new("hold")
+        .unwrap()
+        .initialize_default()
+        .with_organ(OrganType::Bladder);
+
+    {
+        let bladder = patient.find_organ_typed_mut::<Bladder>().unwrap();
+        bladder.volume_ml = bladder.micturition_threshold_ml + 140.0;
+        bladder.urgency = 0.82;
+        bladder.cortical_inhibition = 0.1;
+        bladder.voluntary_hold_command = 0.1;
+        bladder.continence_training = 0.45;
+        bladder.update(0.5);
+    }
+    let baseline = patient.bladder_metrics().unwrap();
+
+    {
+        let bladder = patient.find_organ_typed_mut::<Bladder>().unwrap();
+        bladder.cortical_inhibition = 0.9;
+        bladder.voluntary_hold_command = 0.9;
+        bladder.continence_training = 0.9;
+        bladder.update(0.5);
+    }
+    let hold = patient.bladder_metrics().unwrap();
+
+    assert!(hold.voluntary_hold_fraction > baseline.voluntary_hold_fraction);
+    assert!(hold.cortical_gate_fraction > baseline.cortical_gate_fraction);
+    assert!(hold.cortical_gate_fraction <= 1.0);
+    assert!(hold.voluntary_hold_fraction <= 1.0);
+}
+
+#[cfg(feature = "ffi")]
+#[test]
+fn ffi_exposes_bladder_metrics() {
+    use core::mem::MaybeUninit;
+    use medicallib_rust::ffi::{
+        ml_patient_bladder_metrics, ml_patient_free, ml_patient_new, ml_patient_update,
+        MLBladderMetrics, MLPatient, ML_ERR, ML_OK,
+    };
+    use std::ffi::CString;
+
+    unsafe {
+        let id = CString::new("ffi-metrics").unwrap();
+        let patient: *mut MLPatient = ml_patient_new(id.as_ptr());
+        assert!(!patient.is_null());
+        assert_eq!(ml_patient_update(patient, 0.5), ML_OK);
+        let mut metrics = MaybeUninit::<MLBladderMetrics>::zeroed();
+        let status = ml_patient_bladder_metrics(patient as *const MLPatient, metrics.as_mut_ptr());
+        assert_eq!(status, ML_ERR);
+        ml_patient_free(patient);
+    }
+}

tests/ffi.rs

@@ -29,3 +29,24 @@ fn ffi_errors() {
     let s = medicallib_rust::ffi::ml_patient_summary(std::ptr::null());
     assert!(s.is_null());
 }
+
+#[cfg(feature = "ffi")]
+#[test]
+fn ffi_bloodstream_metrics() {
+    use medicallib_rust::ffi::*;
+    use std::ffi::CString;
+
+    let id = CString::new("ffi-blood").unwrap();
+    let p = ml_patient_new(id.as_ptr());
+    assert!(!p.is_null());
+
+    let mut metrics = std::mem::MaybeUninit::<MLBloodstreamMetrics>::uninit();
+    assert_eq!(ml_patient_update(p, 0.5), ML_OK);
+    let rc = ml_patient_bloodstream_metrics(p, metrics.as_mut_ptr());
+    assert_eq!(rc, ML_OK);
+    let metrics = unsafe { metrics.assume_init() };
+    assert!(metrics.oncotic_pressure_mm_hg > 15.0);
+    assert!(metrics.rbc_mature_fraction > 0.4);
+    ml_patient_free(p);
+}
+