feat(xc): add SCANL meta-GGA functional with FD Laplacian kernel

[dyzheng]

Implement Laplacian-dependent XC functional support enabling the SCANL (SCAN + Laplacian) meta-GGA functional. Key changes:

• Register SCANL functional name with runtime user warning
• Add laplacian_rho() for single-FFT spectral Laplacian computation
• Pass correct lapl_rho to tau_xc/tau_xc_spin (was wrongly using grho)
• Add vlapl output from tau_xc/tau_xc_spin
• Implement FD Laplacian kernel for vlapl potential to avoid |G|^2 amplification that causes SCF divergence (following dyzheng PR Fix(module_xc): compute real density laplacian for meta-GGA functionals #7286)
• Add vlapl stress contribution via density Hessian in G-space
• Guard vlapl processing when functional doesn't depend on Laplacian
• Fix SCAN unit test (pass lapl_rho=0.0, vlapl=0.0)
• Update test CMakeLists with required source files and link libraries

Si2 FCC stress FD validation: SCANL 1.1% error, SCAN 0.0% error. Metallic Al error is pre-existing k-point convergence issue.

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[dyzheng]

The remain issue of this PR: