Three levels: step unpacks only the first two levels; README multipatch nits

step() did 'c, f = self._levels', which raises for three levels before any
launch (root review). The child level is driven from the fine hops. Other
_levels uses are per-level loops or explicit indices. README multipatch
example now defines rest locally and states coarse/fine NaN separately.

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

src/CelerisLab/lbm/refinement_stepper.py

@@ -496,7 +496,7 @@ class FixedPatchStepper:
             raise ValueError("n must be a nonnegative integer")
         with self._lock, self.ctx.activate():
             self._require("ready")
-            c, f = self._levels
+            c, f = self._levels[:2]  # a nested child (third level) runs inside the fine hops
             p = self.patch
             kw = dict(block=self._block, stream=self._stream)
             try: