perf: specialize nth for unindexed property sources

[cdc-as81]

Summary

• Implement ConcretePropertySource::nth directly.
• Keep EntitySetIterator::nth unchanged.
• Add test coverage for skipping through an unindexed property source.

Motivation

Unindexed single-property sampling uses Algorithm L, which repeatedly calls
Iterator::nth. Relying on the default trait implementation produced a
codegen-sensitive regression in the 10,000-entity benchmark.

Specializing nth on ConcretePropertySource preserves the same behavior while
stabilizing the hot path.

Benchmark results

Revision 10k median Change from main 100k change from main
main 4.931 µs
Before fix 6.557 µs +33.0% -2.4%
After fix 4.940 µs +0.2% -1.3%

The fix recovers over 99% of the lost 10k performance while retaining the 100k
improvement.

[github-actions]

Benchmark Results

Hyperfine

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
large_sir::baseline	3.6 ± 0.2	3.4	4.2	1.00 ± 0.08
large_sir::baseline_households	3.5 ± 0.1	3.4	4.1	1.00
large_sir::entities	8.1 ± 0.3	7.6	9.2	2.28 ± 0.12
large_sir::households	8.1 ± 0.7	7.6	11.1	2.27 ± 0.22

Criterion

Regressions (slower)

Group	Bench	Param	Change	CI Lower	CI Upper
indexing	with_query_results_indexed_multi-property_entities		75.367%	74.498%	76.092%
sample_entity	sample_entity_single_property_unindexed	10000	23.152%	21.125%	25.037%
counts	reindex_after_adding_more_entities		10.910%	10.577%	11.235%
large_dataset	bench_match_entity		6.791%	6.589%	6.996%
indexing	query_people_indexed_multi-property_entities		4.408%	3.893%	4.901%
counts	index_after_adding_entities		3.650%	3.413%	3.860%
indexing	query_people_count_indexed_multi-property_entities		2.838%	2.530%	3.152%
indexing	query_people_multiple_individually_indexed_properties_entities		2.512%	2.332%	2.693%
indexing	with_query_results_multiple_individually_indexed_properties_enti		2.316%	2.033%	2.546%
examples	example-births-deaths		2.156%	1.369%	3.232%
sampling	sampling_single_known_length_entities		1.890%	1.519%	2.236%
sampling	count_and_sampling_single_unindexed_concrete_plus_derived_entiti		1.505%	1.410%	1.603%

Improvements (faster)

Group	Bench	Param	Change	CI Lower	CI Upper
large_dataset	bench_query_population_multi_indexed_entities		-36.691%	-37.846%	-35.388%
counts	multi_property_indexed_entities		-23.146%	-23.744%	-22.516%
sample_entity	sample_entity_single_property_unindexed	1000	-18.283%	-18.771%	-17.863%
sample_entity	sample_entity_single_property_indexed	100000	-7.943%	-8.933%	-6.935%
sampling	sampling_multiple_unindexed_entities		-3.727%	-3.873%	-3.605%
examples	example-basic-infection		-3.686%	-4.007%	-3.371%
large_dataset	bench_query_population_derived_property_entities		-3.327%	-4.125%	-2.419%
large_dataset	bench_query_population_indexed_property_entities		-3.186%	-4.024%	-2.357%
sample_entity	sample_entity_whole_population	100000	-2.069%	-2.747%	-1.498%
sample_entity	sample_entity_single_property_indexed	1000	-1.663%	-2.116%	-1.234%
indexing	query_people_count_single_indexed_property_entities		-1.610%	-1.864%	-1.355%
sampling	sampling_single_l_reservoir_entities		-1.401%	-1.703%	-1.134%
counts	multi_property_unindexed_entities		-1.318%	-1.665%	-1.029%

Unchanged / inconclusive (CI crosses 0%)

Group	Bench	Param	Change	CI Lower	CI Upper
indexing	query_people_single_indexed_property_entities		1.988%	-0.512%	4.422%
indexing	query_people_count_multiple_individually_indexed_properties_enti		1.385%	0.410%	2.060%
sample_entity	sample_entity_multi_property_indexed	1000	-1.370%	-1.815%	-0.886%
sample_entity	sample_entity_whole_population	1000	1.339%	0.694%	2.061%
sampling	sampling_single_unindexed_concrete_plus_derived_entities		1.160%	0.827%	1.497%
sampling	sampling_multiple_l_reservoir_entities		-1.135%	-1.431%	-0.830%
sampling	sampling_multiple_known_length_entities		1.056%	0.665%	1.622%
large_dataset	bench_query_population_multi_unindexed_entities		-0.938%	-1.442%	-0.576%
sample_entity	sample_entity_single_property_indexed	10000	0.875%	-0.038%	1.882%
counts	single_property_indexed_entities		0.843%	0.319%	1.337%
indexing	with_query_results_single_indexed_property_entities		-0.732%	-1.122%	-0.263%
sample_entity	sample_entity_multi_property_indexed	100000	-0.654%	-1.108%	-0.166%
sampling	sampling_single_unindexed_entities		0.625%	0.348%	0.870%
large_dataset	bench_filter_indexed_entity		-0.579%	-2.275%	1.144%
sample_entity	sample_entity_multi_property_indexed	10000	-0.579%	-1.109%	-0.058%
sample_entity	sample_entity_single_property_unindexed	100000	0.563%	0.242%	0.813%
sampling	count_and_sampling_single_known_length_entities		0.518%	0.233%	0.802%
large_dataset	bench_filter_unindexed_entity		-0.386%	-3.805%	3.395%
large_dataset	bench_query_population_property_entities		-0.313%	-0.789%	0.291%
counts	concrete_plus_derived_unindexed_entities		0.238%	-0.382%	0.966%
algorithm_benches	algorithm_sampling_multiple_known_length		0.206%	-0.207%	0.722%
algorithm_benches	algorithm_sampling_single_l_reservoir		-0.201%	-0.405%	-0.014%
algorithm_benches	algorithm_sampling_single_known_length		0.176%	-0.487%	0.782%
counts	single_property_unindexed_entities		-0.137%	-0.452%	0.112%
sample_entity	sample_entity_whole_population	10000	0.114%	-0.146%	0.354%
algorithm_benches	algorithm_sampling_single_rand_reservoir		-0.090%	-0.275%	0.063%
algorithm_benches	algorithm_sampling_multiple_l_reservoir		-0.018%	-0.367%	0.313%

Not Compared (no baseline yet)

Group	Bench	Reason
(none)