rtx-fea + rtx-cfd: the single-step seams FSI2 stands on

rtx-fea: NonlinearDynamicAnalysis refactored onto a NonlinearDynamicStepper
- set_nodal_forces on both (the interface load of a coupling subiteration,
  replaceable between steps and between subiterations of one step);
- step(&DynamicState) is a pure function of the start-of-step state and
  the current forces - commits nothing, so a partitioned coupling re-runs
  one Newmark step to the interface fixed point (the piston semantics);
- run() marches through the same stepper: one code path, pinned from both
  ends (linear limit, CSM3, and a new manual-drive == run() assertion);
- new test: a nodal step load oscillates about the *static* nonlinear
  analysis's deflection (cross-code-path, mean within 3%, amplitude 6%),
  with re-run determinism and force-swap sensitivity asserted mid-march
  (a one-step response to a force change is ~ beta dt^2 - the first
  assertion draft demanded 10% and was corrected against the physics).

rtx-cfd: the subiteration seam and the moving no-slip closure
- EmbeddedPisoSolver::snapshot()/restore() (mask + time + init flag; the
  mask is now Clone): re-running a fluid step within a subiteration is
  bit-identical to never having diverted - proven on a moving body with
  cells flipping in the re-run window;
- polygon_interface_velocity: nearest-edge linear interpolation of
  per-vertex velocities, exact for the linear-along-edge boundary data a
  finite-element interface hands over - the no-slip closure that replaces
  FSI1's zero-velocity polygon.

Suites: rtx-fea 567, rtx-cfd 325, rtx-fsi piston+transfer - all green.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Lnyrw33Lu6rUhW42E9KHwq

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

@@ -39,9 +39,10 @@ pub use ale::{
 pub use boundary_conditions::{
     BoundaryCondition, BoundaryConditions, BoundaryLocation, BoundaryType,
 };
-pub use embedded::{EmbeddedParameters, EmbeddedPisoSolver, EmbeddedResult};
+pub use embedded::{EmbeddedParameters, EmbeddedPisoSolver, EmbeddedResult, EmbeddedSolverState};
 pub use embedded_body::{
-    EmbeddedBody, EmbeddedMask, FaceKind, SurfaceForce, SurfaceSample, polygon_signed_distance,
+    EmbeddedBody, EmbeddedMask, FaceKind, SurfaceForce, SurfaceSample, polygon_interface_velocity,
+    polygon_signed_distance,
 };
 pub use flow_field::FlowField;
 pub use piso::{PisoParameters, PisoResult, PisoSolver};