[snowyukitty]

Related to #847.

After an SVG gradient has been rendered, a second dispose() call throws because its element no longer has a parent. This also interrupts Path, Text, and Points disposal when the same gradient is used for both fill and stroke.

Check that the SVG element still has a parent before removing it. Keep the cached element and its stops so rendering the gradient again preserves the existing behavior. Add regression coverage for linear and radial gradients: repeated disposal, already-detached elements, and identical fill/stroke on all three shape types.

This addresses the repeated-disposal part of #847; ownership of a gradient shared by separate shapes remains unresolved.

Validation on e8a637bd:

• npm run build, npm run lint, npm run types:check, npm run docs:generate, and npm run docs:build pass.
• The browser disposal suite passes all 160 assertions. With only the new tests applied to the unchanged base, all 10 added scenarios fail on the null-parent removal.
• Chromium rendering checks pass for linear and radial gradients in SVG, Canvas, and WebGL: repeated disposal followed by rendering preserves the pixels, and subsequent stop-color changes remain visible.
• The complete browser suite reports 947 passing and 11 failing assertions, compared with 903 passing and the same 11 failures on the unchanged base: 10 image-rendering comparisons and one Two.Path.getBoundingClientRect expectation. No additional failures were introduced in this environment.

[jonobr1]

Awww, thank you!