barrier() is not implemented on any backend

[corgab]

Context

CircuitBuilder::barrier() / Gate::barrier() are exposed and serialize a {"type": "barrier"} gate, but bin/python/circuit.py skips it (pass). Braket has no native barrier gate. The methods are currently marked with a @todo.

Options

1. Implement as a compiler fence via Circuit.add_verbatim_box() (the real Braket equivalent). Note this changes semantics — verbatim boxes prevent compilation/optimization across the boundary and have device-support constraints.
2. Document explicitly as a logical/no-op separator (visualization aid only) and keep the no-op.
3. Remove until a backend supports it.

Acceptance

• Decide on one of the above.
• Behaviour matches the docblock and README (README currently notes "not yet executed on any backend").
• If implemented: tests asserting the verbatim box is emitted; if removed: API + tests + docs cleaned up.