Safety limits to precomp, repeater, and polystar handling

Introduce bounds to keep resource use predictable on deeply nested or
oversized inputs:
- Cap precomp nesting depth in repeater processing, render-tree build,
and render walk
- Cap total render-node count for wide precomp graphs
- Clamp repeater copy count to a sane maximum
- Validate polystar point count (reject non-finite/negative)