[adrianptrv]

Closes #640

The MCP tool descriptions now explain when a surface role stays equal to base.

• New SURFACE_ROLE_FALLBACK text in descriptions.ts, used in:
  • the SURFACE notes of the palette tool
  • the surface parameter of create_palette
  • the surfaceColor parameter of create_theme
  • surfaceDefinition in create_custom_palette (replaces the old one-line note)
• It covers both cases:
  • on a white surface, raised and overlay stay equal to base
  • on a black surface, sunken stays equal to base
  • surfaces close to white or black can do the same even when there is a little room left
• It says what to do about it: use borders or elevations, or choose a surface color slightly further from white or black.