MAINT: Faster SQLite fixtures

[Roman Lutz (romanlutz)]

Description

Ordinary unit tests replayed the full Alembic migration history for every sqlite_instance, adding substantial overhead across thousands of tests. Build and validate a pristine SQLite template once per pytest worker, then copy it with SQLite's backup API into a fresh database for each test.

• Keep sqlite_instance function-scoped, with a new real SQLiteMemory instance, engine, results directory, and the existing threaded-session behavior. The template has its own engine and is query-only; it never becomes the shared singleton.
• Restore the previous SQLiteMemory and CentralMemory registrations after each test without clearing unrelated singleton registrations. Fixture teardown owns cleanup of its instances, avoiding process-exit callbacks that would retain thousands of objects. This is a separate lifecycle safeguard, not a separately claimed speedup.
• Leave production constructors, migrations, schema checks, and reset_database() unchanged. Tests that exercise those paths still execute them. No workflow tracing settings, sharding, dependencies, or coverage thresholds change.

Some constructor and identity tests now explicitly request patch_central_database even though their assertions do not query the database. Constructing a target calls CentralMemory.get_memory_instance() in PromptTarget.__init__; those tests previously relied on memory left behind by earlier tests. The added function/class markers remove that test-order dependency without introducing a global autouse fixture.

Local measurements used Windows, Python 3.12.13, SQLite 3.50.4, and unchanged coverage 7.13.1 CTracer settings:

Workload	Before	After
Fixture setup median, no coverage	212.28 ms	1.13 ms
Fixture setup median, CTracer coverage	574.83 ms	2.38 ms
Same 176 existing memory/target/scorer tests, CTracer coverage	167.95 s	8.87 s

Fixture medians use 20 samples after two warmups and measure the complete setup. One-time template setup was measured separately at 272 ms without coverage and 786 ms with coverage per worker. These are local measurements, not an estimate of end-to-end CI savings.

Tests and Documentation

• Added 14 regression cases covering row/schema/temp-table/view/result-directory isolation, semantic schema and Alembic revision fidelity, prior singleton/CentralMemory restoration, unrelated registrations, connection cleanup on success and failure, collectability, real resets, and concurrent writers. The fixture infrastructure has 100% line and branch coverage in a focused run.
• Full make unit-test-cov-xml equivalent using the unchanged Makefile target: 17,107 passed, 124 skipped, 7 subtests passed; 91.96% line coverage, above the unchanged 78% gate. Run on Windows/Python 3.12 with the complete default locked dependencies, including feedgen.
• Four-worker memory/migration/initialization/target/scorer selection: 882 passed, 1 skipped. All affected consumer modules plus regressions: 677 passed on four workers.
• The matched 176-test subset lost 22 incidental constructor/singleton/reset/cleanup coverage lines; all 22 are covered in the passing full suite. No migration revision script lost coverage in that comparison. The configured 90% diff gate passed at 93%, including pre-existing base/origin differences. Patch-only diff coverage is not applicable because this patch changes only tests and documentation.
• Ruff, formatting, the new fixture/regression type checks, documentation validation, and all applicable commit hooks passed. Expanded type checking of the consumer modules reports four pre-existing mock-return diagnostics, reproduced from the unchanged starting commit.
• Updated the contributor guide with fixture ownership and explicit memory-dependency guidance. No notebooks changed, so JupyText was not run. Linux/all-extra CI and other Python versions were not run locally.