Add Terrain Regression Tests

Prompt file imported from akspa0/parp-tools (.codex/prompts/add-terrain-regression-tests.md). Copyright stays with the author.

Add first-party regression coverage for the active terrain pipeline in gillijimproject_refactor.

Requirements:

• Prefer test projects next to the active code under gillijimproject_refactor/src, not under lib, next, or archived folders.
• Prioritize the smallest high-value seam first.
• Good starting seams are Mcal decode variants, StandardTerrainAdapter.ExtractAlphaMaps(), TerrainTileMeshBuilder alpha-shadow packing, and TerrainImageIo atlas roundtrip parity.
• If a real-data integration check is needed, use the fixed development paths from the memory bank and say exactly what still requires manual validation.
• Build the changed production project and the changed test project.
• If the repo has no suitable first-party test project yet, scaffold the minimum viable one instead of writing pseudo-tests in docs.