feat: add set_fills tool for multi-fill stack support

[Ananchai-m]

Motivation

set_fill_color only accepts a single SOLID color and replaces the entire fills array — multi-fill stacks (base color + semi-transparent overlay, two stacked gradients, etc.) couldn't be set via MCP. This blocked testing converters / pipelines that handle multi-fill specifically.

Changes

Adds a set_fills tool accepting an array of paint specs:

set_fills({
  nodeId: "123:456",
  fills: [
    { type: "SOLID", color: { r: 0.85, g: 0.85, b: 0.85 }, opacity: 1 },
    { type: "SOLID", color: { r: 0, g: 0, b: 0 }, opacity: 0.5 },
  ],
})

Each entry takes type (SOLID / GRADIENT_LINEAR), color (or gradientStops for gradients), opacity, blendMode, and visible. Order is bottom-up: index 0 is the bottom-most layer, last entry renders on top — matches Figma's own fills array semantics.

Plugin handler builds the paint objects, validates required fields per type, and assigns to node.fills. Server exposes the tool with a zod schema mirroring the runtime validation.

Tests

Manually verified locally — calling set_fills on a Figma rectangle produces the expected stacked appearance, matches what the user would get from manually pressing + in Figma's Fill panel.

Files changed

• src/talk_to_figma_mcp/server.ts — tool definition + zod schema + CommandParams entry
• src/cursor_mcp_plugin/code.js — setFills() handler + dispatch case