Selectable via STRIX_RUNTIME_BACKEND=podman, with auto-detection of Podman sockets across Linux (rootless/rootful) and macOS (applehv/libkrun podman machine), correct host-gateway hostname (host.containers.internal), and multi-layer socket fallthrough: STRIX_RUNTIME_SOCKET -> DOCKER_HOST -> per-backend auto-detection -> docker.from_env() default.
Adds backend-aware CLI/connection checks (check_docker_installed, check_docker_connection) and 28 unit tests covering backend registry, socket candidate generation, podman machine inspect JSON parsing, and socket fallthrough. Adds make test target wired into make check-all.
Closes#106
