feat: add calendar date utilities

[cdc-as81]

Summary

• Add a day-based Calendar mapping simulation time 0.0 to a Gregorian date.
• Add ContextCalendarExt helpers for date conversion and date-based scheduling.
• Re-export Chrono date types and calendar operations through Ixa's root and prelude.
• Support ISO date serialization without timezone or clock dependencies.

Behavior

• One simulation-time unit represents one calendar day.
• Fractional times map to their containing day using floor semantics.
• Negative simulation times map to dates before the epoch.
• Missing configuration, duplicate configuration, non-finite times, and date-range
  overflow produce explicit errors.
• Existing timeline and scheduling behavior remains unchanged.

[github-actions]

Benchmark Results

Hyperfine

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
large_sir::baseline	3.3 ± 0.0	3.2	3.4	1.00
large_sir::baseline_households	3.4 ± 0.1	3.3	3.6	1.03 ± 0.02
large_sir::entities	8.0 ± 0.6	7.5	10.6	2.43 ± 0.18
large_sir::households	7.4 ± 0.2	7.1	8.0	2.24 ± 0.06

Criterion

Regressions (slower)

Group	Bench	Param	Change	CI Lower	CI Upper
large_dataset	bench_filter_unindexed_entity		24.727%	20.015%	29.640%
large_dataset	bench_match_entity		16.067%	15.789%	16.371%
counts	multi_property_indexed_entities		8.919%	7.717%	10.154%
large_dataset	bench_filter_indexed_entity		8.244%	6.291%	10.222%
sample_entity	sample_entity_single_property_indexed	1000	6.046%	4.384%	7.807%
sampling	sampling_single_unindexed_entities		4.549%	4.473%	4.626%
examples	example-births-deaths		4.538%	3.724%	5.338%
sample_entity	sample_entity_single_property_indexed	10000	4.131%	1.617%	7.446%
sampling	sampling_multiple_unindexed_entities		3.675%	3.562%	3.765%
sample_entity	sample_entity_multi_property_indexed	1000	3.168%	2.651%	3.610%
sample_entity	sample_entity_multi_property_indexed	10000	3.142%	2.627%	3.616%
algorithm_benches	algorithm_sampling_single_rand_reservoir		2.985%	2.604%	3.319%
examples	example-basic-infection		2.902%	1.494%	4.111%
sampling	sampling_single_unindexed_concrete_plus_derived_entities		2.877%	2.518%	3.237%
sample_entity	sample_entity_multi_property_indexed	100000	2.849%	2.300%	3.356%
large_dataset	bench_query_population_indexed_property_entities		2.195%	1.335%	2.942%
indexing	query_people_multiple_individually_indexed_properties_entities		2.166%	1.802%	2.524%
sampling	count_and_sampling_single_unindexed_concrete_plus_derived_entiti		1.260%	1.157%	1.362%

Improvements (faster)

Group	Bench	Param	Change	CI Lower	CI Upper
sample_entity	sample_entity_single_property_unindexed	10000	-22.909%	-23.370%	-22.563%
sample_entity	sample_entity_single_property_unindexed	1000	-21.096%	-21.828%	-20.345%
large_dataset	bench_query_population_property_entities		-13.769%	-16.889%	-11.290%
large_dataset	bench_query_population_multi_indexed_entities		-13.399%	-14.799%	-11.835%
indexing	query_people_single_indexed_property_entities		-11.434%	-11.857%	-10.994%
indexing	query_people_count_indexed_multi-property_entities		-10.014%	-11.231%	-8.773%
counts	single_property_indexed_entities		-6.179%	-7.220%	-5.059%
indexing	query_people_count_multiple_individually_indexed_properties_enti		-5.795%	-6.145%	-5.471%
indexing	with_query_results_multiple_individually_indexed_properties_enti		-4.789%	-5.319%	-4.244%
indexing	query_people_count_single_indexed_property_entities		-4.128%	-4.751%	-3.457%
indexing	with_query_results_indexed_multi-property_entities		-4.067%	-4.578%	-3.417%
counts	multi_property_unindexed_entities		-2.030%	-2.542%	-1.572%

Unchanged / inconclusive (CI crosses 0%)

Group	Bench	Param	Change	CI Lower	CI Upper
large_dataset	bench_query_population_multi_unindexed_entities		1.605%	0.994%	2.290%
sample_entity	sample_entity_whole_population	1000	-1.335%	-2.038%	-0.553%
indexing	with_query_results_single_indexed_property_entities		-1.310%	-1.960%	-0.587%
counts	index_after_adding_entities		-1.190%	-1.903%	-0.651%
sampling	count_and_sampling_single_known_length_entities		-1.116%	-1.564%	-0.727%
sampling	sampling_multiple_known_length_entities		-1.040%	-1.702%	-0.375%
indexing	query_people_indexed_multi-property_entities		1.029%	0.439%	1.637%
large_dataset	bench_query_population_derived_property_entities		0.955%	-0.045%	1.904%
sample_entity	sample_entity_single_property_indexed	100000	0.947%	-1.214%	3.029%
sampling	sampling_single_l_reservoir_entities		-0.785%	-1.330%	-0.340%
counts	reindex_after_adding_more_entities		-0.705%	-1.229%	-0.105%
sampling	sampling_single_known_length_entities		-0.681%	-1.265%	-0.210%
algorithm_benches	algorithm_sampling_single_l_reservoir		-0.616%	-0.898%	-0.360%
sample_entity	sample_entity_whole_population	100000	0.605%	-0.243%	1.392%
counts	single_property_unindexed_entities		0.547%	-0.097%	1.340%
sample_entity	sample_entity_whole_population	10000	-0.435%	-1.091%	0.248%
sample_entity	sample_entity_single_property_unindexed	100000	0.310%	0.021%	0.589%
algorithm_benches	algorithm_sampling_multiple_known_length		-0.304%	-0.618%	0.017%
algorithm_benches	algorithm_sampling_single_known_length		-0.256%	-3.129%	2.778%
algorithm_benches	algorithm_sampling_multiple_l_reservoir		-0.239%	-0.715%	0.266%
counts	concrete_plus_derived_unindexed_entities		-0.168%	-0.812%	0.343%
sampling	sampling_multiple_l_reservoir_entities		-0.064%	-0.202%	0.052%

Not Compared (no baseline yet)

Group	Bench	Reason
(none)