refactor(parallel_grid): pass KPAR into z_distribution as a parameter

[MrLi000001]

Summary

Parallel_Grid::z_distribution() previously read GlobalV::KPAR directly, so its
behaviour depended on global state rather than on its caller. This PR makes kpar
an explicit parameter: the caller Parallel_Grid::init() supplies
GlobalV::KPAR, and the z_distribution() body no longer reads any global variable.

This is a narrow, behaviour-preserving refactor that removes one cross-layer
dependency on GlobalV::KPAR inside source/source_base/parallel_grid.*.

Changes

• source/source_base/parallel_grid.h
  • void z_distribution(void); → void z_distribution(const int kpar);
• source/source_base/parallel_grid.cpp
  • Parallel_Grid::init() now calls this->z_distribution(GlobalV::KPAR);
  • Parallel_Grid::z_distribution(const int kpar) uses kpar for the startp
    allocation and the pool loop instead of GlobalV::KPAR.
  • Added assert(kpar > 0); at the top of z_distribution(). This preserves the
    protection that the caller's assert(GlobalV::KPAR > 0) previously provided
    for startp[0], now that the function can be called with any kpar.

Behaviour / Compatibility

• No functional change: init() passes exactly the same value (GlobalV::KPAR)
  that z_distribution() used to read itself.
• The function body is now free of global reads, so it can be reasoned about and
  tested from its arguments alone.
• No INPUT parameter, documented behaviour, or public interface changes.

Scope Note

This PR intentionally contains only the z_distribution(kpar) change. The
unrelated Parallel_Grid::init split into init_serial / init_parallel
subroutines is kept in a separate PR / branch (refactor-parallel-grid-init) and
is not part of this diff.

Verification

Environment: cmake + make, ENABLE_MPI=ON, ENABLE_LCAO=ON (existing build/).

• Build (passed):

[github-actions]

Agent Governance Check

Severity	Rule	Location	Reason	Suggested action	Exception
warning	Global dependency budget	source/source_base/parallel_grid.cpp:97	Added line introduces 1 GlobalV/GlobalC/PARAM reference(s); PR total added=1, removed=2, net_delta=-1.	Confirm this is a migration-neutral move or partial cleanup, and explain the remaining global dependency rationale.	allowed
warning	PR metadata completeness	pull_request.body	missing sections: Linked Issue, Unit Tests and/or Case Tests for my changes, What's changed?	Fill the PR template with issue linkage, test evidence, and a concise change summary.	allowed
warning	Test evidence review	pull_request.body	Source code changed without test path changes or PR test evidence.	Add focused tests, update a relevant case, or document why tests are not required.	allowed
warning	Documentation sync review	pull_request.body	Source changes have no docs change or explicit no-docs-needed statement.	Add documentation updates for behavior/interface changes, or state why documentation is not required.	allowed