[Feature]: Enable nullable reference types consistently

[samatstariongroup]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the EcoreNetto
• I have searched open and closed issues to ensure it has not already been reported

Description

• Summary: <Nullable>enable</Nullable> is set in only 2 of 5 test projects and in no library project, so null-safety is inconsistent and unenforced where it matters most (the core library).
• Affected files: all */*.csproj (preferably via Directory.Build.props from D1).
• Proposed solution: Enable nullable across the solution (centrally), then resolve resulting warnings incrementally. Pairs with A4/C6.
• Acceptance criteria:
  • Nullable is enabled solution-wide.
  • Build is warning-clean (or warnings tracked as follow-ups).