Complete multi-provider naming migration (SqlClient and legacy artifacts)

[cursor]

Problem

The library supports SQL Server, PostgreSQL, MySQL, and SQLite, but legacy SQL Server naming and metadata remain:

• Core public API is still named SqlClient / IAsyncSqlClient
• AssemblyInfo.cs describes "A simple Sql Server wrapper"
• BulkOperations imports Microsoft.Data.SqlClient directly
• config.json previously included unused ServerType: "MSSQL"
• LINQ infrastructure is still typed against SqlClient in places

This creates confusion for consumers adopting non-SQL Server providers.

Proposed solution

Plan a phased migration:

1. Documentation pass — clarify that SqlClient is provider-agnostic despite the name
2. Add aliases — introduce DbClient / IDbClient as preferred names (keeping SqlClient as obsolete aliases initially)
3. Update metadata — assembly description, XML docs, README terminology
4. Breaking change release — remove obsolete names in a major version

Acceptance criteria

• Migration plan documented (non-breaking → breaking timeline)
• New provider-agnostic names available
• Assembly metadata and public docs no longer describe SQL Server only
• Obsolete attributes guide consumers through rename

Notes

This is a potentially breaking API change. Coordinate with NuGet packaging (#25) and semver strategy.