Fix GhosttyCore scrollback cells rendering with black colors

[hobostay]

Summary

• Fix getScrollbackCell() in @wterm/ghostty unconditionally packing RGB values into fgRgb/bgRgb even when colorFlags indicates no explicit color was set
• This caused scrollback lines to always render with rgb(0,0,0) (black) foreground/background instead of using terminal default colors
• Now matches the same colorFlags check pattern used by getCell()

Before

return {
  char: cell.codepoint || 32,
  fg: DEFAULT_COLOR, bg: DEFAULT_COLOR,
  flags: cell.flags,
  fgRgb: packRgb(cell.fgR, cell.fgG, cell.fgB), // always set → black
  bgRgb: packRgb(cell.bgR, cell.bgG, cell.bgB), // always set → black
};

After

const result = { char: cell.codepoint || 32, fg: DEFAULT_COLOR, bg: DEFAULT_COLOR, flags: cell.flags };
if (cell.colorFlags & 1) result.fgRgb = packRgb(...);
if (cell.colorFlags & 2) result.bgRgb = packRgb(...);
return result;

Test plan

• Verify scrollback lines in Ghostty core render with correct colors instead of black
• Compare scrollback rendering with viewport rendering — colors should match

🤖 Generated with Claude Code

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