rtx-cfd: the embedded-body pieces FSI1 stands on (belongs with b82f307)

EmbeddedMask::traction_at (the per-sample traction factored out of
surface_force, so a coupling loop can load a structure at its own
quadrature points), EmbeddedBody::polygon and the public
polygon_signed_distance (a deformable interface as a vertex list, usable
behind a lock through EmbeddedBody::from_sdf). Left unstaged by mistake
in b82f307 — that commit's FSI1 test needs these to compile.

Co-Authored-By: Claude Fable 5 <[email protected]>

crates/specialized/rtx-cfd/src/solvers/incompressible/mod.rs

@@ -40,7 +40,9 @@ pub use boundary_conditions::{
     BoundaryCondition, BoundaryConditions, BoundaryLocation, BoundaryType,
 };
 pub use embedded::{EmbeddedParameters, EmbeddedPisoSolver, EmbeddedResult};
-pub use embedded_body::{EmbeddedBody, EmbeddedMask, FaceKind, SurfaceForce, SurfaceSample};
+pub use embedded_body::{
+    EmbeddedBody, EmbeddedMask, FaceKind, SurfaceForce, SurfaceSample, polygon_signed_distance,
+};
 pub use flow_field::FlowField;
 pub use piso::{PisoParameters, PisoResult, PisoSolver};
 #[cfg(feature = "cuda")]