[hzqst]

Parallax displacement reverses on SURF_PLANEBACK faces because the face builder negates both texture tangents while leaving UVs in their original texinfo directions. Keep the outward-normal correction, preserve the UV tangent directions, and mirror the corresponding tangent when a decal uses FDECAL_HFLIP or FDECAL_VFLIP.

The production basis builders are shared with the existing real-OpenGL regression suite. It now checks the attached cube's six face orientations and all four decal flip combinations against independent world-ray/height-plane intersections, including five heights, positive/negative/zero scale, and frontal/oblique views. Existing midpoint and material-alignment coverage is retained.

Refs MetaHookSv/MetaHookSv#972. Root-cause investigation. Aggregator gitlink update: MetaHookSv/MetaHookSv#973; merge this component PR first.

Validation:

• Demonstrated failures with the old signs: -Y world U expected 0.394000, actual 0.406006; after fixing face orientation alone, +Y/HFLIP decal U expected 0.406000, actual 0.393982.
• Windows MSVC x86 Debug and Release: Renderer and all 18 test executables build successfully; CTest 19/19 in each configuration, no failures or skips.
• Real GL: 270 six-face world cases + 1,080 six-face/decal-flip cases, retaining 432 midpoint/material/lightmap cases and 24 production shader variants. Intel UHD Graphics 770, OpenGL 4.4.
• Pinned clang-format 23.1.3: 105 files, no differences; git diff --check passes; independent read-only review found no blocking issue.

The GPU regressions use the production basis builders and fragment shader with a test vertex shader. Full in-game visual acceptance has not been performed. This change also corrects the shared basis consumed by normal mapping.