TSL: add dFdx and dFdy warnings

[sunag]

Description

Improves warning and avoids crashing the renderer if dFdx and dFdy are used in non-fragment shader stage.

[Copilot]

Pull Request Overview

This pull request adds runtime warnings when dFdx and dFdy are used outside the fragment shader stage, preventing renderer crashes.

• Introduces a conditional check for non-fragment shader stages
• Logs a warning message and comments out the method string when dFdx or dFdy is encountered

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.36 78.34	336.36 78.34	+0 B +0 B
WebGPU	532.74 148.03	532.9 148.08	+152 B +49 B
WebGPU Nodes	532.21 147.93	532.36 147.98	+152 B +50 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.37 112.21	465.37 112.21	+0 B +0 B
WebGPU	604.38 164	604.53 164.05	+152 B +51 B
WebGPU Nodes	559.37 153.45	559.52 153.5	+152 B +49 B