Bubble k_v and k_g module-level variables not initialized

[sbryngelson]

Bug

In `src/simulation/m_global_parameters.fpp`, module-level scalar variables `k_v` and `k_g` (vapor and gas thermal conductivities for bubble dynamics) are not initialized to `dflt_real` alongside their `bub_pp%k_v` and `bub_pp%k_g` counterparts.

File: `src/simulation/m_global_parameters.fpp`, lines 703-704

Impact: When the thermal bubble model is not used, `k_v` and `k_g` retain whatever value was in memory, potentially causing incorrect behavior if accidentally referenced.

Severity: High — uninitialized module-level variables in bubble physics parameters.

[sbryngelson]

Fix: #1183