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Solved the problem -- this is related to permissions issues stemming from using Perforce. (Perforce has subsequently been taken out back and shot.)

Additionally: When I hook things up again and hit Compile Shader, the shader is displayed correctly in the game preview window, but not in the Shader Forge preview window. When I save, close Shader Forge, and re-open, all connections are broken again.

Thanks for the workaround, works fine!