d2svg: reject padding with negative dimensions

[alixander]

Summary

• preserve valid negative padding as a supported way to crop the viewport
• reject padding that would overflow platform integers or produce a negative SVG width or height
• preserve zero-sized dimensions, which SVG treats as non-rendering rather than invalid

Before this change, for example, d2 --pad=-1000 could succeed while emitting negative width, height, and viewBox dimensions. SVG 2 treats negative width or height as illegal: https://www.w3.org/TR/SVG2/struct.html

Validation

• go test ./d2renderers/d2svg -count=1
• go test -race ./d2renderers/d2svg -run '^TestRenderValidatesPaddingDimensions$' -count=20
• go vet ./d2renderers/d2svg
• GOOS=js GOARCH=wasm go test -c ./d2renderers/d2svg
• go test ./e2etests -run '^TestE2E$' -count=1 -parallel=1
• CLI controls: --pad=-1 renders; --pad=-1000 returns an error

The regression covers a valid negative crop, the exact zero-dimension boundary, the first invalid negative value, and minimum/maximum int64 inputs.