README: minimal three-level (nested child) usage and limits

Doc-only: child coordinates in L1 window cells, three arrays in/out, 1:2:4,
single body-free chain; example reuses the accepted 16/24/24 fixture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -123,7 +123,7 @@ with CudaContext(0) as ctx, FixedPatchStepper(cfg, (8, 6, 24, 18), context=ctx,
 ```
 
 Scope and limits:
-- D2Q9, SRT, FP32, double buffer, unshifted, no LES or forcing; fixed 2:1 patch(es), two levels.
+- D2Q9, SRT, FP32, double buffer, unshifted, no LES or forcing; fixed 2:1 patch(es), two levels (or one nested third).
 - `coarse_boundary="periodic"` (default) or `"slot_reservoir"` (bottom slot, top
   reservoir; patch needs `y0 >= 4`, `y1 <= ny-4`). Periodic mode ignores the wall/slot config fields.
 - Object centers/radii use the coarse **cell-edge** frame (cell `I` spans `[I, I+1)`),
@@ -155,8 +155,28 @@ Scope and limits:
       coarse_g, fine_g = run.download()  # coarse: NaN on every covered cell;
                                          # fine: NaN on every non-ACTIVE cell
   ```
+- Three levels (one nested chain, body-free): add `child=(x0, y0, x1, y1)` in
+  **L1 window cells** (the patch's fine grid including its 4-cell ghost band) and
+  `child_block`, with `fine_block` and `base_block`. L2 must sit inside L1's
+  ACTIVE cells, away from L1's ghost band and transfer footprint (checked before
+  any GPU allocation). `initialize` and `download` take/return three arrays; each
+  level is NaN on its non-ACTIVE (and child-covered) cells. One coarse step = 2 L1
+  steps = 4 L2 steps. One L1 patch only, no bodies/Jets.
+
+  ```python
+  cfg = LBMConfig(nx=16, ny=16, viscosity=.1)
+  rest = np.asarray(compute_feq_d2q9(1.0, 0.0, 0.0), np.float32)[:, None, None]
+  with CudaContext(0) as ctx, FixedPatchStepper(
+          cfg, (4, 4, 12, 12), context=ctx, fine_block=(4, 4), base_block=(4, 4),
+          child=(8, 8, 16, 16), child_block=(4, 4)) as run:
+      shapes = (run.patch.coarse_shape, run.patch.fine_shape, run.child_patch.fine_shape)
+      run.initialize(*(np.broadcast_to(rest, (9, *s)) for s in shapes))  # 16², 24², 24²
+      run.step(32)
+      coarse_g, l1_g, l2_g = run.download()
+  ```
 - Not available: per-link surface exchange/force readout in blocked mode,
-  checkpoints, moving bodies, bodies/Jets with several patches, dynamic refinement, `Simulation`
+  checkpoints, moving bodies, bodies/Jets with several patches or three levels,
+  several patches per level with nesting, dynamic refinement, `Simulation`
   integration. Validation so far is storage/operator equivalence and short runs,
   not accuracy, convergence or long-time stability.