test(performance): isolate storage workloads

[gbaudrit]

Summary

Moves relational storage performance workloads out of the default functional test lanes while preserving a bounded deterministic contention check. Resolves #303 as part of #298.

Changes

• Adds a dedicated Agentstration.Performance.Tests project and points the performance solution at it.
• Keeps a small SQLite concurrent-write correctness smoke in Agentstration.Web.Tests.
• Fixes optimistic-version handling in the workload and enriches JSON reports with workload, runtime, OS, throughput, latency, error, conflict, and retry data.
• Adds path-aware PostgreSQL smoke artifacts plus a weekly/manual full SQLite and PostgreSQL workflow.
• Renames the Flow SQL pagination test file to describe its query contract and documents the baseline policy.

Validation

• Affected Release projects build with 0 warnings and 0 errors.
• SQLite contention correctness smoke: 1/1 passed.
• Flow SQL pagination/filtering contract: 1/1 passed.
• Performance solution SQLite smoke: 1/1 passed.
• SQLite workload at 40 operations / concurrency 8: 0 errors, 0 conflicts; JSON inspected.
• SQLite full workload at 500 operations / concurrency 16: 0 errors, 0 conflicts; JSON inspected.
• PostgreSQL execution is configured for service-backed CI and was not run locally.
• Relevant documentation updated.
• No unnecessary breaking change introduced.
• Desktop and mobile smoke test not applicable (test and CI organization only).

Breaking changes

None.