Extract reusable core library

[rndv1]

Summary

• add a new SnapForge.Core class library for reusable rendering/domain code
• move models, presets, themes, rendering, and utilities out of the CLI project
• update the CLI and tests to consume SnapForge.Core
• update docs to describe the new project structure and Web GUI roadmap

Why

This prepares SnapForge for a Web GUI without duplicating rendering logic or turning the CLI project into a shared application layer.

Verification

• dotnet format --verify-no-changes --no-restore
• dotnet build --no-restore --configuration Release
• dotnet test --no-build --configuration Release --verbosity normal
• dotnet pack src/SnapForge.Cli/SnapForge.Cli.csproj --no-build --configuration Release --output artifacts/packages
• installed SnapForge from the local .nupkg into a temporary tool path and generated a sample card
• git diff --check origin/main...HEAD