Read exchanged q-state occupation in non-KPAR PW EXX - #7992

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 #7426

Unit Tests and/or Case Tests for my changes

• Commands run:
  • git apply --check of this patch on op_pw_exx.cpp at develop cfd4dcf8
• Result summary: apply succeeds. After the change the non-KPAR scalar is occupation(iq,m)/nqs and still multiplies get_pw(m_iband, iq).
• Checks not run, with reason: full CMake/CTest of ABACUS is not run here; ACE/KPAR is the default route and is not edited. The edited site is the non-KPAR act_op gather of wg.

What's changed?

wg stores k-weighted occupation. Non-KPAR PW EXX/HSE read (*wg)(this->ik, m_iband) while the exchanged orbital is get_pw(m_iband, iq) and the denominator is already wk[iq]. That mixes the current-k numerator with the q denominator.

This change reads (*wg)(iq, m_iband), so the scalar is the occupation of the exchanged state. The KPAR path already gathers (*wg)(iq_loc_spin, m_iband). KPAR wk[iq] is pool-local and is not rewritten.

Fixes #7426.

Governance Notes

• INPUT/docs changes: none. No INPUT parameter change.
• Core module impact: OperatorEXXPW::act_op non-KPAR occupation gather only. Hamilt/ESolver/HSolver/ElecState/Psi interfaces unchanged.
• Exceptions requested: none.