fix: Add Ehrenfest example: XXZ spin chain with anisotropy parameter (12 qubits) (closes #991)

spec/examples/xxz_spin_chain_12q.cbor.hex

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spec/examples/xxz_spin_chain_12q.md

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+# XXZ Spin Chain (12 Qubits)
+
+## Physics Description
+
+The XXZ spin chain is a one-dimensional quantum spin model with anisotropic interactions, described by the Hamiltonian:
+
+$$H = J \sum_{i=0}^{N-2} (\sigma_i^x \sigma_{i+1}^x + \sigma_i^y \sigma_{i+1}^y + \Delta \sigma_i^z \sigma_{i+1}^z)$$
+
+Where:
+- $J$ is the coupling strength (set to 1.0)
+- $\Delta$ is the anisotropy parameter (set to 1.0 for the Heisenberg point)
+- $N$ is the number of qubits (12 in this example)
+
+This model exhibits rich physics including quantum phase transitions as the anisotropy parameter $\Delta$ is varied:
+- $\Delta < -1$: Ferromagnetic Ising phase
+- $-1 < \Delta < 1$: XY phase with algebraic decay of correlations
+- $\Delta > 1$: Antiferromagnetic Ising phase
+
+## Parameters
+
+- **Qubits**: 12
+- **Coupling strength (J)**: 1.0 GHz·rad
+- **Anisotropy parameter (Δ)**: 1.0 (Heisenberg point)
+- **Evolution time**: 10.0 μs
+- **Trotter steps**: 1
+- **Required T1**: 100.0 μs
+- **Required T2**: 100.0 μs
+
+## Expected Results
+
+At the Heisenberg point ($\Delta = 1$), this model exhibits SU(2) symmetry and is expected to show antiferromagnetic correlations. The ground state has total spin $S_{total} = 0$ for even $N$, with alternating spin expectation values along the z-axis.
+
+The energy per bond is expected to approach $E_0/N \approx -0.443$ GHz·rad in the thermodynamic limit, with finite-size corrections for the 12-qubit chain.
+
+## Circuit Properties
+
+- **Depth**: ~50 gates
+- **CX gates**: 22 (nearest-neighbor)
+- **Single-qubit gates**: 24 (hadamard and phase)
+- **Qubit connectivity**: Linear chain (0-1-2-...-11)