Polish public readiness docs

[Courtland9777]

Summary

• Replace the status template with a concise factual current-status page.
• Align the MIT license copyright holder with package metadata.
• Note in the changelog that current work remains unreleased and package-dry-run only.

Validation

• git diff --check
• dotnet format IntervalTree.Net.sln --verify-no-changes with temporary NuGet caches after the default implicit restore hit cache permissions
• dotnet restore IntervalTree.Net.sln with temporary NuGet caches
• dotnet build IntervalTree.Net.sln --configuration Release --no-restore with temporary NuGet caches
• dotnet test IntervalTree.Net.sln --configuration Release --no-build with temporary NuGet caches
• dotnet pack src/IntervalTree.Net/IntervalTree.Net.csproj --configuration Release --no-build with temporary NuGet caches

No release, NuGet publication, visibility change, or career-material approval is included in this PR.