Guard U2F workflows to prevent overlapping HAL borrows

[benma]

Motivation

• Prevent multiple detached U2F workflow futures from running concurrently and mutably aliasing the single global HAL, which can cause undefined behavior or crashes when a host triggers overlapping U2F requests.

Description

• Add a global WORKFLOW_IN_PROGRESS: AtomicBool in src/rust/bitbox02-rust-c/src/u2f_c_api.rs and use compare_exchange at the start of rust_workflow_spawn_unlock and rust_workflow_spawn_confirm to reject new spawns while a workflow is active, and clear the flag when the async future finishes.

Testing

• Ran formatting and test commands used in the project: cd src/rust && cargo fmt (failed: rustfmt/cargo-fmt not installed), ./scripts/docker_exec.sh make -j run-rust-unit-tests (failed: docker not available), and cd src/rust && cargo test -p bitbox02-rust-c --no-run (failed: bindgen not in PATH); no unit tests could be executed in this environment but the change is limited and compiles in-tree where toolchain prerequisites are met.

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Codex Task