fix: bake node color into canvas texture to preserve emoji colors

[chiefcll]

Summary

• Bake node.color into CanvasTextRenderer.fillStyle instead of hardcoding white — text glyphs render in the correct color directly on the canvas, and emoji glyphs retain their native browser colors since they bypass fillStyle
• Force shader tint to white (with worldAlpha) for canvas text nodes so the GPU doesn't double-tint the already-colored texture
• Re-render canvas text on color change by overriding the color setter in CoreTextNode to invalidate layout when color changes on a canvas text node

Why

CanvasTextRenderer hardcoded context.fillStyle = 'white', producing a texture with white text + natively colored emoji. The shader then multiplied the entire texture by node.color. For dark text colors (light theme), this crushed emoji to near-black silhouettes.

Closes #1

Trade-off

Gradient-tinted canvas text (non-uniform colorTl/Tr/Bl/Br) will no longer produce a gradient since the shader corners are forced to white. This is acceptable because canvas text color is now baked into the texture, and gradient text is an edge case that's better served by the SDF renderer.

Test plan

• Render <Text color={0x0f172aff}>🔥 hello</Text> with CanvasTextRenderer — emoji should retain native colors, text should be dark
• Render <Text color={0xf1f5f9ff}>🔥 hello</Text> — both text and emoji should appear correctly (regression check)
• Change color dynamically — text should re-render with the new color
• Verify alpha/worldAlpha fading still works on canvas text nodes
• SDF text rendering is unaffected

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