Some engine assets are loaded directly by C++ renderer code with no
corresponding reference in any XML template, actor, or data file that
checkrefs.py can statically trace. Files in mod layers that override
these paths were incorrectly reported as unused.
Introduce source/tools/entity/cpp_hardcoded_assets.json to maintain
the list of such VFS paths alongside their C++ source location and a
description. checkrefs.py reads this JSON via add_cpp_hardcoded_assets()
and marks every matching file in all loaded mod layers as a known-used
root, so mod overrides are no longer flagged.
Initial entries:
art/textures/terrain/types/water/coastalWave.png
art/textures/terrain/types/water/foam.png
Both are loaded unconditionally by WaterManager::LoadWaterTextures().
Fixes 2 false-positive unused-file warnings for mod/ water textures.
