post_process: Q-criterion-only cases access unallocated finite-difference coefficient arrays #1564

Symptom: a post_process case enabling only qm_wrt (no vorticity/schlieren/liutex) reads unallocated fd_coeff_* arrays — crash or garbage depending on compiler.

Mechanism: the allocation guards in s_initialize_derived_variables_module (src/post_process/m_derived_variables.fpp:44/48/52 on master) test omega_wrt/schlieren_wrt/liutex_wrt but not qm_wrt, while the call guards in m_start_up.fpp:216/220/224 (and the Q-criterion computation at :631) include qm_wrt.

Introduced: with the feature in #79 (fd07dfe65, 2023-01-30, @hyeoksu-lee) — the call guards gained qm_wrt, the allocation guards never did (liutex was later added to both; qm never was). Tagging @hyeoksu-lee for context.

Fix: #1556 makes the allocation guards mirror the call sites exactly.