refactor: extract MD statistics as pure functions

[lijianing-sudo]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #... (refactoring #7: extract MD temperature, kinetic energy, and stress statistics as pure functions)

Unit Tests and/or Case Tests for my changes

No new tests are added — the behavioral surface is unchanged. All existing MODULE_MD tests pass because current_temp() and compute_stress() are preserved as thin wrappers that delegate to the new pure functions internally. The floating-point computation paths are identical.

What's changed?

Extract temperature, kinetic energy, and stress statistics from the MD_func namespace into pure functions that return explicit structs, instead of writing results through mutable reference parameters.

New file: source/source_md/md_statistics.h

• MDKineticState — holds kinetic and temperature
• MDStressState — holds t_vector (ionic kinetic tensor) and stress (total stress)
• FIREProjection — reserved for future FIRE refactoring (not used yet)

source/source_md/md_func.h

• Add #include "md_statistics.h"
• Declare calc_kinetic_state() and calc_stress_state() — pure, no side effects

source/source_md/md_func.cpp

• Implement calc_kinetic_state() and calc_stress_state()
• current_temp() becomes a wrapper: calls calc_kinetic_state(), writes back to ref params
• compute_stress() becomes a wrapper: calls calc_stress_state(), assigns stress
• kinetic_energy() and temp_vector() are unchanged

source/source_md/nhchain.cpp — 4 call sites updated in first_half() and second_half():

// Before
t_current = MD_func::current_temp(kinetic, ucell.nat, frozen_freedom_, allmass, vel);
MD_func::compute_stress(ucell, vel, allmass, cal_stress, virial, stress);

// After
MDKineticState kstate = MD_func::calc_kinetic_state(ucell.nat, frozen_freedom_, allmass, vel);
kinetic   = kstate.kinetic;
t_current = kstate.temperature;
if (cal_stress) {
    MDStressState sstate = MD_func::calc_stress_state(ucell, vel, allmass, virial);
    stress = sstate.stress;
}

source/source_md/msst.cpp — 4 call sites updated in setup() and second_half().

Backward compatibility: md_base.cpp, verlet.cpp, run_md.cpp, and md_func_test.cpp still call the old current_temp()/compute_stress() interfaces, which continue to work unchanged.

Any changes of core modules? (ignore if not applicable)

No. All changes are confined to source/source_md/.

[mohanchen]

This refactoring idea is quite interesting. Feel free to move forward with your implementation.