Add .NET 8 class library scaffolding and GitHub Packages publish workflow

[Copilot]

Repo was initialized with no project structure or CI. Adds the minimal scaffolding to build, pack, and publish mdz-core as a private NuGet package to GitHub Packages.

Changes

src/mdz-core/mdz-core.csproj

• SDK-style, net8.0, nullable/implicit-usings enabled
• PackageId=mdz-core, GeneratePackageOnBuild=false
• Drop existing .cs source files directly into src/mdz-core/

.github/workflows/publish.yml

• Triggers on push to main and v*.*.* tags
• Version derived from tag (v1.2.3 → 1.2.3); falls back to 1.0.0 on branch push
• Restore → Build → Test (skipped if no tests/ csproj found) → Pack → Publish
• Authenticates to https://nuget.pkg.github.com/{owner}/index.json via built-in GITHUB_TOKEN — no extra secrets needed

.gitignore

• Adds out/ to exclude dotnet pack artifacts

# Workflow trigger summary
on:
  push:
    branches: [main]
    tags: ["v*.*.*"]

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