Add GitHub Actions CI to catch build breaks, SQL install script errors, and DuckDB schema regressions before manual testing.

Scope:

• ci.yml: Build all 5 .NET projects + run DuckDB schema tests on every PR/push to dev
• sql-validation.yml: Install all SQL scripts on SQL Server 2017/2019/2022/2025 matrix, validate all objects exist (path-filtered to install/**)
• Lite.Tests: 6 xUnit tests for DuckDB schema initialization, version, idempotency, indexes

PR: #75