//! Contract test: nested `begin_phase()` calls panic.
//!
//! Phases are flat. A nested `begin_phase()` previously corrupted the outer
//! phase's slab (later masked by a depth counter, which itself had failure
//! modes — a panic between matched calls left the depth orphaned). The
//! library now enforces the flat-phase contract with an assertion in
//! `begin_phase()`: any second begin while a phase is active is a panic.
//!
//! This test pins that behavior so a future depth-counter regression is
//! caught at `cargo test`.
static A: ZkAllocator = ZkAllocator;