feat: add parametric Pauli rotation gates

[mahbub570]

Adds R_XX, R_YY, R_ZZ, and R_PAULI per clifft Pauli rotation spec: exp(-i * alpha * pi/2 * PP).
Encoded as SPP + parametric tag, same pattern as TPP → SPP[T].

Now we can use like-

from tsim import Circuit

# two-qubit special cases
c = Circuit("""
    R_XX(0.25) 0 1
    R_YY(0.5) 0 1
    R_ZZ(0.3) 0 1
""")
 
c = Circuit("R_PAULI(0.25) X0*Y1*Z2")
 
c = Circuit()
c.append("R_XX", [0, 1], arg=0.25)
c.append("R_PAULI", "X0*Y1", arg=0.25)

# inverse works
assert (c + c.inverse()).to_matrix() ≈ I

Closes: #142

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve SPP_DAG direction during Clifford expansion

When a user supplies a half-pi parametric SPP_DAG[...] instruction, Circuit.stim_circuit expands it using only the tag angle and ignores instr.name. For example, SPP_DAG[R_PAULI(theta=0.5*pi)] X0*Y1 is the inverse Pauli rotation, but this maps theta=0.5 to SPP instead of SPP_DAG, changing the unitary returned by the public stim_circuit property. Account for SPP_DAG by negating the effective theta or swapping the expanded SPP/SPP_DAG gates.

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[chatgpt-codex-connector]

Keep theta when inverting existing SPP_DAG rotations

For circuits that already contain a parametric SPP_DAG[...] instruction, Stim's inverse flips it to SPP[...] with the same tag, which is already the correct inverse under parse_stim_circuit's is_dag sign handling. This block then negates theta and always appends SPP, so Circuit.from_stim_program(stim.Circuit('SPP_DAG[R_PAULI(theta=0.25*pi)] X0')).inverse() represents the original rotation instead of its inverse. The theta should only be negated for the post-inversion SPP_DAG case, not for SPP.

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