Example usage guards: reject negative --steps; document blocked command

Primary documented command (example docstring and README) now passes
--fine-block 16 20; omitting it keeps the dense default unchanged.

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

examples/fixed_patch_circle.py

@@ -1,8 +1,9 @@
 """Public API demo: fixed 2:1 patch with a stationary, optionally actuated circle.
 
 From this checkout, after checking the selected physical GPU is idle:
-    PYTHONPATH="$PWD/src" CUDA_VISIBLE_DEVICES=0 python examples/fixed_patch_circle.py
+    PYTHONPATH="$PWD/src" CUDA_VISIBLE_DEVICES=0 python examples/fixed_patch_circle.py --fine-block 16 20
     ... --speed -0.002 --fine-block 16 20 --mode slot_reservoir
+Omit --fine-block for the (equivalent) dense fine storage.
 
 A short smoke run (default 128 coarse steps): it shows setup, stepping and
 fluid-only diagnostics, not a developed flow, accuracy or long-time stability.
@@ -74,6 +75,8 @@ def main():
                         help="blocked fine storage; must divide the whole fine allocation (32 x 40 here)")
     parser.add_argument("--steps", type=int, default=128)
     args = parser.parse_args()
+    if args.steps < 0:
+        parser.error("--steps must be nonnegative")
     root = Path(__file__).resolve().parents[1]
     if Path(CelerisLab.__file__).resolve() != root / "src/CelerisLab/__init__.py":
         raise RuntimeError(f"Set PYTHONPATH to {root / 'src'}")