feat: entity framework core schema generation

[Stephanvs]

What changed

• adds a separate D2Lang.EntityFrameworkCore NuGet package targeting .NET 8 and .NET 10
• converts EF Core relational models into typed D2 SQL tables, constraints, and foreign-key connections
• adds AddD2Schema<TContext> for automatic startup generation and ToD2Diagram() for direct use
• resolves relative output paths from the host content root, writes atomically, and skips unchanged files
• adds documentation, CI package validation, release publishing, and parser-backed tests

Why

Database diagrams should remain synchronized with the EF Core model instead of being maintained manually. The integration reads EF metadata without connecting to the database and produces source that can be rendered by the D2 CLI.

Validation

• multi-target Release build: 0 warnings, 0 errors
• tests: 37/37 passed, including real d2 validate
• NuGet package validation and .nupkg/.snupkg inspection passed
• clean .NET 8 and .NET 10 consumer builds passed
• NuGet vulnerability audit found no vulnerable dependencies
• simulated 2.0.0 release preserved the correct d2lang-cs package dependency

Dependency

PR #5 is merged; this branch is based on its merge commit.