docs(bmad): Add Hierarchical Subsystems (FR48) to PRD, Arch, Epics and Sprint

_bmad-output/planning-artifacts/architecture.md

@@ -283,6 +283,32 @@ impl Compressor<Disconnected> {
 - Extensible pour composants custom (e.g., Ejecteur de Robert)
 - AHRI 540 coefficients intégrés dans struct Compressor
 
+### Hierarchical Subsystems (MacroComponents)
+
+**Decision:** Wrapper Pattern matching the `Component` trait
+
+**Core Pattern:**
+```rust
+struct MacroComponent {
+    internal_system: System,
+    port_mapping: HashMap<PortId, InternalLocation>, /* e.g., Exposes 'Condenser Water In' */
+}
+
+impl Component for MacroComponent {
+    fn compute_residuals(&self, state: &SystemState, residuals: &mut ResidualVector) {
+        // Delegates or flattens computation to internal_system
+        self.internal_system.compute_residuals(state, residuals);
+    }
+    // ... maps external ports to internal boundary ports ...
+}
+```
+
+**Rationale:**
+- Allows users to build reusable blocks (like a full Chiller, Air Handling Unit)
+- Mimics Modelica/Simulink ecosystem composability
+- The global solver treats `MacroComponent` exactly like a basic Component, preserving zero-cost abstractions
+- `SystemState` flattening ensures equations are solved simultaneously globally, avoiding nested numerical solver delays.
+
 ### Fluid Properties Backend
 
 **Decision:** Trait abstraction with multiple backends
@@ -794,7 +820,7 @@ pub trait Solver {
 | Feature | FRs | Location |
 |---------|-----|----------|
 | Component Modeling | FR1-FR8 | `crates/components/src/` |
-| System Topology | FR9-FR13 | `crates/solver/src/system.rs` |
+| System Topology | FR9-FR13, FR48 | `crates/solver/src/system.rs` & `macro_component.rs` |
 | Solver Engine | FR14-FR21 | `crates/solver/src/strategies/` |
 | Inverse Control | FR22-FR24 | `crates/solver/src/inverse/` |
 | Fluid Properties | FR25-FR29 | `crates/fluids/src/` |