v0.19.2

This patch moves Hegel's data-generation engine out of the hegeltest crate and into libhegel (the hegel-c crate). hegeltest now drives the engine entirely through libhegel's C ABI, exactly like the other language bindings. For Rust users this is an internal change — the public API is unchanged — but it keeps us honest: hegel-rust can no longer accidentally depend on engine internals that aren't exposed through the C ABI.

For libhegel C-ABI consumers (such as hegel-go) it adds and hardens some surface:

• A new hegel_settings_backend(settings, backend) together with the hegel_backend_t enum (HEGEL_BACKEND_AUTO / HEGEL_BACKEND_DEFAULT / HEGEL_BACKEND_URANDOM) lets you choose the engine's source of randomness. HEGEL_BACKEND_AUTO is the default: it picks URANDOM automatically when running under Antithesis and the seeded, reproducible PRNG (DEFAULT) otherwise. This auto-detection now lives in the engine rather than in each binding.

• libhegel no longer panics on invalid arguments, so it is now correct to build it with panic = "abort". hegel_target returns HEGEL_E_INVALID_ARG (with a message in hegel_last_error_message) for a non-finite score or a duplicate label, and the id-taking primitives (hegel_collection_more, hegel_collection_reject, hegel_pool_add, hegel_pool_generate, hegel_state_machine_next_rule) return HEGEL_E_INVALID_ARG for an unknown id instead of aborting the process. Only genuine internal-invariant violations still panic.