R8-c: the 3D flag's interface — deformed-plate body (host + device φ/ub) and the conservative load transfer onto a Hex20 plate

- embedded3/plate.rs: PlateSurface (mid-surface on span stations) and the
  host evaluation of DeviceSdf (phi_host / velocity_host / is_flag_host),
  expression for expression e3_geom.cu; a span-uniform plate is the
  polyline capsule to the bit; first-order spanwise-slope correction.
- e3_geom.cu / device/geom.rs: plate branch of geom_phi_at and
  body_velocity (nst = 0 keeps the polyline path unchanged).
- cutwall.rs / exchange.rs: the load loops observed through a sink (sums
  unchanged); interface.rs: Mask::cut_wall_loads (every summand of
  cut_wall_force with its foot), HexPlate (R8-b's Hex20 lattice numbering),
  consistent point-force transfer conserving force and moment to round-off,
  locate() for the transpose, mid_surface() for the fluid body.
- flag test: RTX_E3_FLAG_BODY=plate, _STATIONS, _TWIST, _TRANSFER(_EVERY,
  _CSV, _NODAL); all default off.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/mod.rs

@@ -15,8 +15,10 @@ pub mod export_vtk;
 pub mod field;
 pub mod grid;
 pub mod impose;
+pub mod interface;
 pub mod loads;
 pub mod maskupdate;
+pub mod plate;
 pub mod poisson;
 pub mod reconstruct;
 pub mod step;
@@ -27,6 +29,8 @@ pub use cut::CutGeometry;
 pub use export_vtk::write_vtk;
 pub use field::Field;
 pub use grid::Grid;
+pub use interface::{HexPlate, LoadKind, PlateTransfer, WallLoad};
 pub use loads::SurfaceForce;
+pub use plate::PlateSurface;
 pub use step::{Boundaries, Fluid, Parameters, Side, Solver, StepResult};
 pub use wall::{FaceKind, Mask, WallScheme};