Add Hirshfeld partitioning to weights generation

[awvwgk]

Add XCWeightAlg::Hirshfeld as a host molecular-weight partitioning scheme for host (OMP/MPI).

$$ w_A(r) = \rho_A^0(|r - R_A|) / \sum_B \rho_B^0(|r - R_B|) $$

Use a Gaussian shape-function where each atom contributes a normalized Gaussian shape and the normalization function is the sum of reference charges multiplied with the shape.

$$ \rho_A^0(r) = Z_A (\alpha_A / \pi)^{3/2} \exp(-\alpha_A |r - R_A|^2) \qquad\text{with}\quad \alpha_A = 0.5 / r_A^2 $$

Timings for some toy systems:

Threads	Atoms	Points/atom	Becke	SSF	LKO	Hirshfeld
1	12	256	361	314	463	84.8
1	64	128	10292	8340	6304	434
1	128	64	42538	25932	11930	878
1	256	32	177689	87608	20427	1888
4	12	256	89.8	76.4	121	22.2
4	64	128	2580	2610	1611	112
4	128	64	10677	8285	3021	223
4	256	32	45250	29550	5169	489

[Copilot]

Pull request overview

Adds a new molecular partitioning scheme (XCWeightAlg::Hirshfeld) to GauXC’s host weight generation pipeline (Reference/Default host LWD), including analytical first-derivative support and C-API exposure, with new/updated tests to validate correctness and enum correspondence.

Changes:

• Introduce XCWeightAlg::Hirshfeld in both C++ and C enums and wire it through the host reference local work driver.
• Implement Hirshfeld partition weights and first-derivative (direct + contracted) reference routines on host.
• Extend unit tests and C-API tests to cover Hirshfeld weight generation and factory usage.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/weights.cxx	Adds Hirshfeld reference-data generation hook and new unit tests validating Hirshfeld invariants (normalization, translation, permutation).
tests/weights_host.hpp	Extends host weight test switch to call the Hirshfeld reference implementation.
tests/weights_generate.hpp	Extends reference-data generator to support Hirshfeld.
tests/weight_derivative_test.cxx	Adds Hirshfeld to analytical derivative test dispatch and adds fdiff sections for it.
tests/c_api_test.cxx	Verifies C/C++ enum correspondence for Hirshfeld; adds factory creation and modify-weights coverage for Hirshfeld in C-API.
src/xc_integrator/local_work_driver/host/reference/weights.hpp	Declares Hirshfeld host reference routines (weights + derivatives + contraction).
src/xc_integrator/local_work_driver/host/reference/weights.cxx	Implements Hirshfeld weights and derivative routines on host.
src/xc_integrator/local_work_driver/host/reference_local_host_work_driver.cxx	Wires Hirshfeld into host “Reference/Default” local work driver switches.
include/gauxc/enums.hpp	Adds XCWeightAlg::Hirshfeld to the public C++ enum.
include/gauxc/c/enums.h	Adds GauXC_XCWeightAlg_Hirshfeld to the public C enum.

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