style: apply rustfmt across all crates and demos

Consistent formatting pass: line wrapping, import sorting, trailing
whitespace removal, let-chain indentation, merged derive attributes,
and unsafe block reformatting.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

demos/rtx-cardiosim-demo/src/monodomain.rs

@@ -6,8 +6,8 @@
 //! where V is transmembrane voltage, D is diffusion tensor, and Iion is ionic current.
 
 use cardiosim_shared::{
-    ConductivityParams, HeartMesh, IonicModel, SimulationConfig, StimulationProtocol,
-    Vector3D, VoltageField,
+    ConductivityParams, HeartMesh, IonicModel, SimulationConfig, StimulationProtocol, Vector3D,
+    VoltageField,
 };
 
 use crate::CardioSimError;
@@ -25,7 +25,7 @@ pub struct MonodomainSolver {
 
 impl MonodomainSolver {
     /// Create a new monodomain solver.
-    #[must_use] 
+    #[must_use]
     pub fn new(config: &SimulationConfig) -> Self {
         Self {
             ionic_model: config.ionic_model,