refactor(bitmap): reshape API — enum class, factory functions, BitmapConvert, .cpp split

[JeanPhilippeKernel]

Reshapes the Bitmap API for clarity and scalability.

Changes

• BitmapType and BitmapFormat are now enum class (uint8_t backing) — no namespace pollution
• Depth field renamed to Layers (more accurate: 6 faces for CubeMap, 1 for Texture2D)
• Overloaded constructors replaced by two named factory functions:
  • Bitmap::Create(w, h, layers, ch, fmt, type, slab) — allocates zeroed buffer
  • Bitmap::FromData(w, h, layers, ch, fmt, type, data, slab) — allocates and copies
• Conversion algorithms moved to BitmapConvert namespace as free functions:
  • BitmapConvert::EquirectToCross
  • BitmapConvert::CrossToCubemap
• BitmapPixel struct removed — FaceCoordToXYZ is now a file-local static in Bitmap.cpp
• Implementation split into Bitmap.cpp — header is declarations only

Call sites updated

• EnvironmentMapImporter.cpp
• RenderResourceManager.cpp
• AssetCodec.cpp
• bitmap_test.cpp