renderSVG: limits against denial of service

[matmen]

Image.renderSVG could be made to use unbounded time or memory with small crafted SVGs. This PR adds three limits against that.

input	before	after
"Billion laughs": nine levels with ten <use> each (1 kB, 10⁹ rectangles)	killed (memory)	Error after 0.57 s
stroke-dasharray="0.0001" on a long line	killed (memory)	stroked solid, 12 ms
100,000 nested <g>	Maximum call stack size exceeded	renders, 144 ms

Changes (src/render/svg.ts)

• <use> limit: elements rendered through <use> references are counted, and the render throws after 100,000. librsvg has a similar limit. Only instanced elements count, because only they can multiply; direct elements are bounded by the file size, so large plain SVGs are unaffected.
• Dashes: a dash pattern that would produce more than 1,000,000 dashes strokes the path solid, as Skia does.
• Deep nesting: the pass that collects IDs and style sheets uses an explicit stack instead of recursion. Document order is preserved, so the first ID still wins.

Reference cycles (use → use, gradients, clip paths, masks) were already caught. The tests confirm that.

Testing

• Fuzzing: 540 damaged variants of the test SVGs (truncated, or with junk such as broken references, huge numbers and tiny dash arrays inserted) produce no hangs and no slow cases.
• Existing renders: all test targets are unchanged.
• tests/regressions.js: covers billion laughs, the fine dash pattern and deep nesting.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YJgRK5qpwJEbHPuFv6kgAQ