Add SKP occlusion test

[nongio]

Summary

• add integration test recording SKP for nested layers
• verify occluded layer increases recorded operations

Testing

• cargo test --test skp_occlusion -- --nocapture

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https://chatgpt.com/codex/tasks/task_e_68ab073bcfe4832dad0180d6965dc387

[github-actions]

Benchmark for 207570b

Click to view benchmark

Test	Base	PR	%
engine::append/1	116.3±64.13µs	79.1±42.32µs	-31.99%
engine::append/10	114.3±62.84µs	78.4±41.19µs	-31.41%
engine::append/100	114.4±62.86µs	78.1±41.80µs	-31.73%
engine::append/1000	77.8±41.44µs	78.0±41.71µs	+0.26%
engine::remove/1	0.6±0.00ns	0.6±0.01ns	0.00%
engine::remove/10	0.6±0.01ns	0.6±0.01ns	0.00%
engine::remove/100	0.7±0.27ns	0.7±0.27ns	0.00%
engine::remove/1000	1248.3±101.33µs	1276.1±104.02µs	+2.23%
engine::update/1	31.7±0.96µs	31.7±0.78µs	0.00%
engine::update/10	79.6±1.29µs	80.6±2.39µs	+1.26%
engine::update/100	301.4±3.12µs	302.6±2.94µs	+0.40%
engine::update/1000	2.3±0.23ms	2.3±0.22ms	0.00%

[github-actions]

Benchmark for 0208669

Click to view benchmark

Test	Base	PR	%
engine::append/1	78.2±41.46µs	77.0±40.93µs	-1.53%
engine::append/10	79.7±42.82µs	76.7±41.00µs	-3.76%
engine::append/100	78.4±41.68µs	76.7±40.91µs	-2.17%
engine::append/1000	78.3±41.77µs	76.8±41.11µs	-1.92%
engine::remove/1	0.6±0.01ns	0.6±0.01ns	0.00%
engine::remove/10	0.6±0.01ns	0.6±0.01ns	0.00%
engine::remove/100	0.7±0.27ns	0.7±0.27ns	0.00%
engine::remove/1000	1249.7±107.90µs	1251.5±102.32µs	+0.14%
engine::update/1	33.2±0.91µs	32.6±1.00µs	-1.81%
engine::update/10	112.6±18.43µs	92.3±12.28µs	-18.03%
engine::update/100	303.8±4.26µs	304.1±3.35µs	+0.10%
engine::update/1000	2.4±0.24ms	2.3±0.24ms	-4.17%