[ycherkes]

Summary

• Add opt-in UseRaffinertExpressions(options => options.Transform(...)) overloads, without touching Core or introducing a production optimizer dependency.
• Execute ordered transforms after Raffinert expansion and before the original EF Core compiler/native parameter extraction for ordinary sync/async queries.
• Keep callbacks in immutable per-context options rather than internal service-provider global state; keep separate interceptor as expansion-only fallback.
• Document behavior, limitations and side-effect-free requirement of external optimizer.

Integration coverage

• Add actual Linq.Expression.Optimizer 1.0.39 as an integration-test-only NuGet dependency and pass ExpressionOptimizer.visit into the pipeline, execute SQLite query containing Raffinert Invoke and a Boolean expression and assert SQL/results.
• Verify transformer order, options isolation and explicit null-result failure.

Verification

Not run locally: .NET SDK is unavailable in execution environment. CI and integration suite must pass before ready for review. Review external optimizer's captured-closure evaluation versus native EF parameter semantics for production usage.

Separate from draft #15 (Condition.Balance()): no change to Core.