Fix(dftu-pw): fix DFT+U for PW basis (nspin=1/2/4) and occupation mat…

[dyzheng]

…rix mixing

• Relax DFT+U nspin validation to support nspin=1/2/4 for PW basis
• Encapsulate Plus_U with typed accessors (get/set_locale, get_orbital_corr, get_hubbard_u, is_locale_initialized, mark_locale_dirty, enable_mixing)
• Rewrite cal_occ_pw for nspin=1/2/4 with correct becp indexing
• Add DFT+U occupation matrix mixing via Broyden method in Charge_Mixing
• Fix DFT+U locale double-counting when kpar>1
• Fix spin-channel selection: use isk[ik] instead of ik>=nk/2 in cal_occ_pw
• Extract setup_pw_dftu_indices() from cal_ps_dftu
• Propagate ld_psi for correct GEMM strides when ngk[ik] < npwx
• Restructure eff_pot_pw layout: nspin=2 uses split spin_up|spin_down
• Add get_eff_pot_pw_spin(isk) for nspin-aware access
• Add becp_ready caching in OnsiteProjector
• Update dftu_lcao.cpp for new Plus_U accessor interface
• Add unit tests for PW DFT+U (dftu_pw_test.cpp)
• Update dftu_io.cpp to use new accessors and onsite.dm format

Reminder

• I have read AGENTS.md and docs/developers_guide/agent_governance.md.
• I have linked an issue or explained why this PR does not need one.
• I have added adequate unit tests and/or case tests, or explained why not.
• I have listed the exact verification commands run and their results.
• I have described user-visible behavior changes, including INPUT parameter changes.
• I have explained core-module impact for ESolver, HSolver, ElecState, Hamilt, Operator, Psi, or other source/ changes.
• I have requested any needed governance exception below.

Linked Issue

Fix #

Unit Tests and/or Case Tests for my changes

• Commands run:
• Result summary:
• Checks not run, with reason:

What's changed?

• Example: brief summary of the user-visible or developer-facing change.

Governance Notes

• INPUT/docs changes:
• Core module impact:
• Exceptions requested: