fix(dagcircuit): avoid copying global_phase into layer DAGs

[92meharali]

Problem

BasicSwap could inflate global_phase when rebuilding a circuit from
DAGCircuit.serial_layers(), even when no swaps were inserted (#16271).

Root Cause

layers() and serial_layers() created each layer DAG with
copy_empty_like(), which copied the parent global_phase. Composing those
layer DAGs back together accumulated the phase once per layer.

Solution

Reset global_phase to zero on layer DAGs returned by layers() and
serial_layers(), matching the existing pattern in separable_circuits().

Tests Added

• test_serial_layers_do_not_copy_global_phase
• test_layers_do_not_copy_global_phase
• test_preserves_global_phase (BasicSwap, no swaps)
• test_preserves_global_phase_when_swaps_inserted (BasicSwap, with swap)

Fixes #16271

AI/LLM disclosure

• I used Cursor (agent) to help implement and test-plan this fix.
• I reviewed the diff, traced the failure through serial_layers() → compose(),
  and verified the fix matches the existing separable_circuits() pattern.

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[Cryoris]

#16362 is an existing and more complete fix, I would give the author a chance to finish that work first. If that doesn't work out we can consider this one instead.

[92meharali]

Thanks @Cryoris understood. I'll hold off and wait for #16362 to land first. Happy to help review or iterate if needed once that PR is active again.