v0.2.1 — prob_floor on the continuous engine (delta_p_min knob)

dcl_core v0.2.1 is a single-feature patch: the prob_floor
parameter on dcl_core.core.CausalSession. It exists to unblock
dcl-delta-p-min — the core column of its 4-cell grid
(exp_09_dp_min_floor) cannot run without a continuous-engine
delta_p_min knob — by giving that investigation a pinnable artifact
(dcl_core @ git+...@v0.2.1).

This is not v1.0.0. The formal API freeze remains gated on
dcl-delta-p-min finishing and co-releases with Paper III.

Semver: PATCH. prob_floor is purely opt-in and backward-compatible
— prob_floor=None (default) is bit-for-bit the v0.2.0 engine; no
re-exported symbol is added or removed.

What prob_floor does

from dcl_core.core import CausalSession
s = CausalSession(lattice, node, omega, prob_floor=1e-6)