query: Improve cancellation, context ownership, and discovery

[erenken]

Summary

• Describe the library as a query-object pattern in both READMEs and package metadata.
• Replace EF InMemory sample/tests with SQLite relational execution and correct SQL-translatable sample filtering.
• Add cancellation-aware list/scalar execution without removing existing query or scalar signatures.
• Inject scoped typed DbContexts, and verify two independent databases route queries correctly and dispose their contexts at scope end.
• Preserve discovery across all currently loaded assemblies; add optional assembly selection, deduplicate registrations, and exclude unconstructable open generic types.
• Expand README setup, multiple-context, discovery, compatibility, testing, and sample-run instructions.

Reviewer Notes

• Existing scalar queries and custom repositories remain compatible through default interface implementations. Their legacy fallbacks only check cancellation before execution; implementing the new overload is necessary to forward cancellation to in-flight EF operations.
• Default scanning still includes inherited/custom context and repository interfaces across loaded assemblies. Newly loaded plugins require another registration call. List/scalar query objects remain caller-created for runtime criteria.
• SQLite testing validates actual SQL translation but does not replace production-provider integration tests. The sample's case normalization demonstrates ASCII case-insensitive matching, not identical Unicode behavior across providers.

Validation

• Release build for .NET 8 and .NET 10: zero warnings/errors.
• Tests: 29 passed per target framework (58 total).
• Sample application successfully runs on both frameworks.
• NuGet package and symbol package creation succeed.
• Git diff whitespace validation passes.