Long vehicle tunnel connected to Control Room's Cake Walk has remarkably different lightmaps before/after BSP switches

[BinToss]

TODO: Investigate if this also affects B40: Assault on the Control Room

Describe the bug
Just like its predecessor in SPV2, this forerunner tunnel that connects two BSPs has a jarring change in lighting when transitioning from one BSP to the other. This is because the BSPs' lightmaps are rendered separately and under differing lighting conditions.

This is the most noteworthy example of the lightmap-switch disparity caused by Tool.exe's lightmap pre-rendering pipeline.

In the short-term, we may need to employ a costly workaround: join a map's BSPs so they create one group of geometry, unwrap the geometry in a way that coincides with each BSPs separate UV maps, and render one, big, very-high-resolution lightmap that is then cut and cropped for each BSP or by loading specific parts of the lightmap to memory to save on memory usage...a somewhat modern feature the engine probably doesn't have.

Alternatively, we may need to manually blend one or both lightmaps in image editing software (e.g. Photoshop, GIMP) and then forcefully replace the lightmap tag in the development environment (bypass the HEK/OS_HEK) or inject it into the spv3c40.yelo cache file, thereby incurring a risk of map corruption.

To Reproduce
Steps to reproduce the behavior:

1. Start Two Betrayals
2. Work your way down into the "drain" tunnels of the Cake Walk with or without a vehicle
3. As you progress through the tunnels, the BSP will switch from the Control Room BSP to its neighbor. At this point, the lightmap will also switch, but the brightness and color of the pre-rendered lightmaps are jarringly different.

Expected behavior
"Overlapping" lightmaps should use the same colors and brightness in "transition" areas so they blend together in a believable fashion.

Screenshots
Near or at the location in the following images...