BackgroundLayer does not render per-cell bgColor from CellStyle

[witqq]

Problem

BackgroundLayer fills the entire canvas with a single theme.colors.background color. It does not read cellData.style.bgColor to render per-cell backgrounds.

Current behavior (v0.1.0 and v0.2.1)

rc.ctx.fillStyle = rc.theme.colors.background;
rc.ctx.fillRect(0, 0, rc.canvas.width, rc.canvas.height);

No per-cell background iteration exists in this layer.

Expected behavior

After the canvas-wide fill, BackgroundLayer should iterate visible cells and render individual backgrounds for cells that have style.bgColor defined:

// After canvas-wide fill:
for (const cell of visibleCells) {
  if (cell.style?.bgColor) {
    ctx.fillStyle = cell.style.bgColor;
    ctx.fillRect(cellX, cellY, cellW, cellH);
  }
}

Impact

Without per-cell background rendering, applications must implement custom layers that:

1. Fill opaque white to erase engine text
2. Apply the desired bgColor on top
3. Re-render text with correct textColor

This creates triple rendering overhead and fragile workarounds.

Versions affected

• @witqq/spreadsheet 0.1.0
• @witqq/spreadsheet 0.2.1