Add an extensible ordered D2 model

[Stephanvs]

What changed

• introduces an ordered D2Statement document abstraction
• adds safe generic scalar/block properties and multiline comments
• adds an explicitly named D2RawStatement escape hatch
• preserves mixed insertion order in diagrams, shapes, and connections
• supports connection property blocks such as style, link, and tooltip
• models layers, scenarios, and steps with nested boards
• materializes input enumerables and exposes read-only statement collections

Why

Modern D2 features need an extensible representation that does not require a new constructor parameter for every language addition. Ordering is also semantically significant for sequence diagrams.

Validation

• dotnet test: 24/24 passed
• parser-backed d2 validate: passed
• C# 10 / netstandard2.0 compatibility checked with the package branch's IsExternalInit shim
• git diff --check: passed

Stack dependency

This PR intentionally targets feature/serialization-safety and contains only one additional commit. Merge PR #4 first, then retarget this PR to main.