Fixes #11705 Add bg_color and fg_color options to GhosttyRenderStateRowCellsData that resolve the final RGB color for a cell, flattening the multiple possible sources. For background, this handles content-tag bg_color_rgb, content-tag bg_color_palette (looked up in the palette), and the style bg_color. For foreground, this resolves palette indices through the palette; bold color handling is not applied and is left to the caller. Both return GHOSTTY_INVALID_VALUE when no explicit color is set, in which case the caller should fall back to whatever default color it wants (e.g. the terminal background/foreground).