Add airfoil-surface wall arrays (kinematic wall shear + surface pressure)

Each train/val/test .npz now carries a per-wall-face surface table: wall_xy,
wall_normal, wall_shear (kinematic tau_w/rho), wall_p (kinematic surface
pressure), wall_length, wall_cell. mesh.h5 files gain the matching /wall group.
OOD split omitted for now (raw cases regenerated separately) — documented in
the README.

README.md

@@ -67,6 +67,44 @@ y ∈ [−1.5, 1.5]   (1.5c above and below)
 
 `N` varies per case (typically 50,000 – 150,000 cells inside the bounding box).
 
+### Airfoil-surface arrays
+
+In addition to the volume point cloud, each file carries a **separate surface
+table** with one row per airfoil wall face (`M` rows, `M ≈ N_wall`, typically a
+few hundred). These live on the airfoil surface itself, unlike `is_wall` which
+marks the first layer of volume cells sitting just off the wall. Rows are
+ordered by wall-face index and are mutually aligned.
+
+| Key | Shape | dtype | Description |
+|---|---|---|---|
+| `wall_xy` | `(M, 2)` | float32 | Wall-face-center coordinates (same frame as `x, y`) |
+| `wall_normal` | `(M, 2)` | float32 | Unit surface normal, pointing from the wall into the fluid |
+| `wall_shear` | `(M, 2)` | float32 | Kinematic wall shear stress vector `τ_w / ρ` (m²/s²) |
+| `wall_p` | `(M,)` | float32 | Kinematic surface pressure (m²/s²) |
+| `wall_length` | `(M,)` | float32 | Face edge length (for surface integration) |
+| `wall_cell` | `(M,)` | int64 | Owner cell index — links each face back to the volume cloud |
+
+> **Availability:** the surface arrays are present for the `train`, `val`, and
+> `test` splits. They are not yet included for the `ood` split (its raw
+> OpenFOAM cases are regenerated separately) — check with
+> `"wall_xy" in np.load(path)` before use.
+
+Quantities are **kinematic** (divided by density, consistent with `p`), matching
+OpenFOAM's `wallShearStress` function object. Multiply by ρ for physical units.
+Skin friction and pressure coefficients follow directly:
+
+```python
+import numpy as np
+
+data = np.load("NACA2412_p3.5_2.0e5.npz")
+u_mag = float(np.hypot(data["u_init"][0], data["v_init"][0]))
+q = 0.5 * u_mag**2                      # kinematic dynamic pressure
+cf = np.linalg.norm(data["wall_shear"], axis=1) / q
+cp = data["wall_p"] / q
+# Chordwise wall-shear sign flags separation (τ_w,x < 0 → reversed flow):
+tau_x = data["wall_shear"][:, 0]
+```
+
 ### Loading a sample
 
 ```python

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