Add runnable history/state/rendering boundary check

[standardgalactic]

Adds the smallest runnable conformance artifact for the boundary between canonical history, replay-derived semantic state, and replaceable rendering.

The new standard-library Python runner:

• starts from the existing SPHIST/1 replay envelope;
• derives state by fresh replay;
• projects that state into canonical renderer input;
• exercises two deliberately different deterministic renderers;
• verifies that rendering changes neither the SPHIST/1 digest nor semantic state.

The check remains separate from the cross-language conformance matrix because renderer agreement is not a kernel invariant. run_all.sh invokes it as its own stage, and the flat-experiments README documents the distinction.

Verified locally with python3 experiments/flat/run_layer_separation.py, the existing python3 experiments/flat/run_python.py suite (15/15 passing), sh -n experiments/flat/run_all.sh, python3 -m py_compile experiments/flat/run_layer_separation.py, and git diff --check.