R4-m: RTX_FSI2O_NODE_CSV — per-node accepted load and displacement along the flag every coupled step

Each wetted node's (dx, dy, fx, fy) grouped bottom / tip / top (sidecar
<path>.nodes.csv: group, node, reference x, y, arc length) plus the
centreline uy at s = 1/4 (mean of the two straddling nodes) and 1/2.
Default off; gate on the R4-l line slice 6.0 -> 6.2 s (CPU V-cycle,
floor 7.5e-8, rtol 1e-3): main vs branch knob-off vs knob-on march CSV
and step CSV byte-identical (md5 70dcf484..., dd535386...); the node
sums equal the step CSV's fx_nodal / fy_nodal.

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

crates/specialized/rtx-fsi/tests/fsi2_harness/mod.rs

@@ -11,6 +11,7 @@
 #![allow(dead_code)] // several test crates share this; each uses a subset
 
 pub mod march;
+pub mod node_csv;
 pub mod overset;
 pub mod overset_march;
 pub mod replay;