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Summary of changes

Fixes #469

phaseshift_gate expanded p, phaseshift and u1 into h; rx(θ); h. A single rz(θ) is the same gate up to a global phase, so it now emits that. A user-written p(pi/4) then rebases to a single t under CLIFFORD_T, and p(pi/2) to s, instead of five gates.

Changes:

• src/pyqasm/maps/gates.py: phaseshift_gate returns one rz(theta). This also shortens csx and rccx, which call it.
• docs/gate_decompositions.md: updated the Phaseshift section and its source link.
• Tests: new regression tests for QASM 3 p/phaseshift/u1, QASM 2 p/u1, Clifford+T rebase of p(pi/4)/p(pi/2)/phaseshift/u1, and full csx/rccx unitaries. Updated the expectations in test_depth.py, test_device_qubits.py and the _check_phase_shift_gate_op helper that hardcoded the old three-gate form.
• CHANGELOG.md entry.

On the global phase: controlled phase gates don't go through this function. ctrl @ p and cp/cphaseshift/cu1 use cphaseshift_gate. Controlled custom gates pass their controls down to each body statement. ctrl @ gphase with two or more controls becomes cp. So the dropped phase stays global. The csx and rccx unitaries are checked exactly in the new tests.

Test plan

• New tests fail on main (12 failures on current main, which includes Decompose ccx, id and p when rebasing to ROTATIONAL_CX / CLIFFORD_T #468) and pass with the change.
• pytest tests: 1033 passed, 3 skipped.
• black, isort and pylint pass on the changed files. mypy reports only the missing-stubs error in modules/base.py that's already on main.

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