R8-a: span-uniform coupled FSI2 on embedded3 — DeviceStep snapshot/restore, operator-route load sink, harness fsi2_embedded3.rs

- embedded3 DeviceStep::snapshot/restore (step/device/snapshot.rs): the start-of-step
  device fields + the moving body's host records; a restore rebuilds the R6 persistent
  geometry/classification, the predictor tables, the hierarchy and the guess basis.
  Only called by the new harness (existing paths untouched).
- exchange::set_load_sink: every contribution of the operator load route (p W, wall
  shear, diffusive/convective exchange) with position/component/part; None by default,
  the routes' sums unchanged (slab ny 62 CSV byte-identical to r6_1/base_slab62.csv).
- rtx-fsi feature cuda (= rtx-cfd/cuda) and tests/fsi2_embedded3.rs: the 2D rtx-fea flag
  (35x2 Quad8, TL, Newmark 0.7) drives the capsule's centreline per coupled step; the
  route's contributions are distributed onto the wetted nodes per unit span; Aitken
  subiterations each re-run the fluid step from the snapshot.

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

crates/specialized/rtx-fsi/Cargo.toml

@@ -21,5 +21,9 @@ futures = { workspace = true }
 rtx-cfd = { workspace = true }
 rtx-fea = { workspace = true }
 
+[features]
+# R8-a: the coupled FSI on the embedded3 device fluid (`tests/fsi2_embedded3.rs`).
+cuda = ["rtx-cfd/cuda"]
+
 [lints]
 workspace = true