feat(organs): add bloodstream organ and patient coupling

Introduce OrganType::Bloodstream and new organ module, exporting
Bloodstream, PerfusionState, and MetabolicState.

- Patient:
  - initialize_default now attaches a bloodstream model
  - add with_bloodstream() builder and with_organ support
  - update() couples bloodstream with heart, lungs, brain, spinal cord,
    kidneys, liver, pancreas, stomach, intestines, gallbladder,
    esophagus, bladder, and spleen
  - bloodstream ingests hemodynamics, respiratory exchange, renal,
    hepatic, splenic, and nutrient feedback; propagates perfusion and
    metabolic signals back to organs
  - blood metrics (SpO2, hemoglobin, hematocrit, glucose) can be driven
    by bloodstream

- Signals:
  - Lungs: add O2 delivery, CO2 elimination, V/Q ratio
  - Liver: add detox and ammonia clearance
  - Kidneys: add plasma volume, urea excretion, erythropoietin

- FFI:
  - add ML_ORGAN_BLOODSTREAM (Rust and C header) and mapping in
    ml_patient_organ_summary

- Examples/Tests:
  - demo monitors "Bloodstream"
  - add tests for bloodstream coupling and organ discovery
  - lib tests updated to include Bloodstream

Rationale: centralize systemic transport and inter-organ homeostasis for
richer physiology simulation and expose it to C consumers via FFI.

ffi/medicallib.h

@@ -91,6 +91,11 @@
  */
 #define ML_ORGAN_SPLEEN 12
 
+/**
+ * Organ code for `OrganType::Bloodstream`.
+ */
+#define ML_ORGAN_BLOODSTREAM 13
+
 /**
  * Opaque patient handle type for C consumers. Wraps a heap-allocated `Patient`.
  */