feat: add Object.create_if_absent atomic insert

[eldonm]

Summary

• Add portable Database.insert_if_absent / InsertIfAbsentResult across SQLite, Postgres, MongoDB, JsonDB, DynamoDB, and cache/observable wrappers.
• Add Object.create_if_absent that never routes through upsert save(), returns (entity, created), and rehydrates the stored winner on conflict.
• Conflict target is primary id only in v1; SQLite uses INSERT OR IGNORE (not OR REPLACE).
• Bump to 0.0.20 with changelog, stability, and entity-reference docs.

Test plan

• pytest tests/db/test_insert_if_absent.py tests/core/test_create_if_absent.py
• CI full suite on this PR
• Confirm save() upsert tests still green

Made with Cursor

[github-actions]

Benchmark comparison

Threshold: ±25% (informational, does not block merge)

benchmark	baseline (s)	current (s)	delta	status
tests/benchmarks/test_deferred_save_benchmarks.py::test_bench_deferred_save_batched_100	0.016629	0.020178	+21.3%	OK
tests/benchmarks/test_deferred_save_benchmarks.py::test_bench_immediate_save_100	0.015861	0.018830	+18.7%	OK
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_batched_saves_500	0.731977	3.131758	+327.8%	REGRESSION (+327.8%)
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_count_empty_query	1.586718	9.236494	+482.1%	REGRESSION (+482.1%)
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_count_filtered	1.274647	4.377951	+243.5%	REGRESSION (+243.5%)
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_find_filtered	0.728067	3.294064	+352.4%	REGRESSION (+352.4%)
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_save_throughput	0.001973	0.007014	+255.5%	REGRESSION (+255.5%)
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_count_empty	0.184939	0.219028	+18.4%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_count_fallback_via_regex	0.248176	0.289107	+16.5%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_count_pushdown	0.212034	0.233636	+10.2%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_find_fallback_via_regex	0.216706	0.264057	+21.8%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_find_pushdown	0.176563	0.232817	+31.9%	REGRESSION (+31.9%)
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_sort_limit_pushdown	0.215163	0.250423	+16.4%	OK