chore: add setup.sh with mac vs cuda branching

setup.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [ ! -d .venv ]; then
+  echo "Creating .venv (Python 3.11)…"
+  python3.11 -m venv .venv
+fi
+
+source .venv/bin/activate
+python -m pip install --upgrade pip
+
+if [[ "$(uname)" == "Darwin" && "$(uname -m)" == "arm64" ]]; then
+  echo "Apple Silicon detected — installing Mac requirements"
+  pip install -r requirements.txt -r requirements-mac.txt
+else
+  echo "Non-Mac platform — installing CUDA-path requirements"
+  pip install -r requirements.txt
+fi
+
+echo "Setup complete. Activate with: source .venv/bin/activate"