Support drive composition with different phases

[alessandro-santini]

Changes:

• drive.py: phase is now stored internally as a ConstantWaveform, composed via the same CompositeWaveform mechanism already used for amplitude/detuning. >> no longer requires equal phases. .phase still returns a float when the whole drive has a uniform phase; raises ValueError if segments differ (use compilation instead of reading .phase directly in that case). draw() now also plots the phase waveform.
• execution/compilation_functions.py: basic_compilation now emits one PulserPulse per contiguous same-phase run (via new _phase_groups helper) instead of always exactly one pulse. For any drive with a uniform phase this still produces exactly one pulse, so behavior is unchanged.
• tests/test_drive.py: updated the test that asserted the old NotImplementedError.

[github-actions]

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