chore: modernize nuget package support

[Stephanvs]

What changed

• targets netstandard2.0, net8.0, and net10.0
• adds deterministic builds, portable PDBs, Source Link, and .snupkg output
• validates the package against published version 1.0.0
• adds repository/package metadata and normalized package paths
• adds global.json with controlled SDK roll-forward
• uses 1.1.0-local for local packs while allowing release workflows to override the version

Why

Targeting only .NET 10 unnecessarily limits a string-generation library, while the previous hard-coded 1.0.0 version made subsequent publishing unsafe.

Validation

• all three library TFMs build cleanly
• solution tests pass
• baseline package validation passes
• default and simulated release packs generated .nupkg and .snupkg
• clean package inspection confirmed all target assemblies and valid Source Link mappings

Stack dependency

This PR targets feature/serialization-safety so its multi-target build uses the netstandard-compatible serializer without a mechanical D2Text conflict. Merge PR #4 first, then retarget this PR to main.