feat(circuits): add 3-qubit gates (Toffoli, Fredkin)

[ddri]

The current gate set is limited to 1-qubit and 2-qubit gates. Missing:

• Toffoli (CCX) — controlled-controlled-NOT. Used in quantum arithmetic, Grover's algorithm oracles, and some error correction schemes.
• Fredkin (CSWAP) — controlled-SWAP. Used in quantum multiplexers and some quantum ML circuits.

Implementation notes

• Add ccx(control1, control2, target) and cswap(control, target0, target1) methods to Circuit in marqov/circuits.py
• Add to the gate map in each converter: to_braket(), to_qiskit(), to_cirq(), to_openqasm()
• Add to each importer: from_braket(), from_qiskit(), from_cirq(), from_openqasm()
• Add tests in tests/test_circuits.py — gate application, roundtrip conversion for each framework
• Update the canonical gate set table in CONTRIBUTING.md §1

References

• Qiskit: CCXGate, CSwapGate
• Braket: CCNot, CSwap
• Cirq: CCNOT, CSWAP

[siddharm]

Does this require an upstream change to quantumflow.stdgates.stdgates_2q.py? CCX and CSWAP can be decomposed into combinations of the existing canonical 2-and-1 qubit gates, but it is a quite complex combination.