Add the Phase(gamma) unitary with exact naming, structural control, and hardware lowering

[ciaranra]

Stacked on #668 (the branch includes its commits; merge #668 first).

What

One new unitary, Phase(gamma), with one rule: on an operand set S of zero, one, or two qubits it multiplies the amplitude by exp(i gamma) on the subspace where every qubit in S is |1> and leaves everything else alone.

• S = {} is a global phase (the scalar exp(i gamma)).
• S = {q} is diag(1, exp(i gamma)), exactly U(0, 0, gamma).
• S = {c, t} is diag(1, 1, 1, exp(i gamma)).
• |S| > 2 is refused at construction.
• Controlling a Phase on S by a qubit c is structurally Phase on S + {c}.

This is layers 1 and 2 of the Phase(gamma) design (representation and hardware lowering). No GateType, byte-level command, simulator fast path, or ingress rewiring yet; those follow once the benchmark decides the command shape.

API (pecos-core)

• Unitary::Phase { gamma: Angle64, num_qubits: u8 } and UnitaryRep::phase_gate(gamma, qubits).
• UnitaryRep::control(self, c) -> Result<UnitaryRep, ControlError>: structural for Phase; refuses a control already in S, a third operand, a non-Phase rep, and a descriptor whose operand count disagrees with its qubit list.
• controlled_rotations::lower_phase(gamma_radians, &[QubitId]) -> Vec<Gate>: {} lowers to no gates (hardware cannot see a scalar; the caller keeps it), {q} to U(0, 0, gamma), {c, t} through lower_cphase.
• Dense conversion builds the diagonal from the rule directly (no half-angle, so negative angles need no representative choice).
• is_clifford reads the shared exact Clifford-angle table (try_simplify_rotation): one qubit at multiples of pi/2, two qubits at 0 and pi. to_named_gate returns a name only when the matrices are exactly equal: I, SZ, Z, SZdg, T, Tdg on one qubit, CZ on two. Adjoint, is_hermitian, to_pauli_string, is_pauli_equivalent/try_to_pauli, Unitary::is_pauli/try_to_pauli, to_clifford_rep, the hardware decomposition, the phase() accessor (a zero-operand Phase is the scalar itself), and circuit-diagram rendering handle Phase.

Fix found on the way

lower_cphase halved the source f64 before Angle64 reduction, so at lambda = 2pi (mod 4pi) the RZZ(-pi) and RZ(+pi) legs both stored HALF_TURN and the lowering executed as -I instead of I (QASM/PHIR cp(2pi) on dev). CPhase is 2π-periodic, so the angle is now reduced to (-pi, pi] first; every halved angle then lies in (-pi/2, pi/2] and the lowering is exact for every input. CRZ/CRX/CRY are 4π-periodic and cannot use that reduction; their documented ±1 at theta = 2pi (mod 4pi) is intrinsic to the two-gate form and is filed as #670 (the exact fix is a zero-operand Phase from the ingress, which this PR makes possible).

Tests

crates/pecos-simulators/tests/phase_gate_contract.rs is the specification: the matrix follows the rule at nine angles including ±2pi, pi, 3pi/2, for S = {}, {0}, {1}, {0,1}, {1,0}; single-qubit Phase equals U(0,0,gamma) entrywise; the lowering composed in the dense path and executed on StateVecSoA equals the rule entrywise (1e-12); control() composes and refuses as specified; Clifford membership and exact naming follow the angle table. Unit tests cover the control() error variants, lower_phase on {}, the T/Tdg names, the Pauli views, and the zero-operand phase().

Follow-ups filed

• to_named_gate names rotations up to global phase but Phase exactly; split or document the semantics #669: to_named_gate names rotations up to global phase (RZ(pi) -> Z) while Phase names exactly; split or document.
• CRZ/CRX/CRY lowering is -1 at theta = 2pi mod 4pi; emit a zero-operand Phase once the ingress has it #670: CRZ/CRX/CRY ±1 at theta = 2pi (mod 4pi).

Verification

cargo fmt --all -- --check; cargo clippy --workspace --all-targets -- -D warnings (default and --all-features lanes); pecos rust test --profile debug (full recipe); just lint check. Mutation: removing the lower_cphase reduction fails both lowering tests in the contract. Two independent reviews (fresh-context and cross-model) ran on the diff; every finding is fixed or refuted in this description.