release: promote main version surface to 0.4.0

[InitusNovus]

Summary

• promote the merged main branch version surface from 0.4.0-alpha.1 to stable 0.4.0
• align package metadata, Api.version(), root READMEs, and NuGet READMEs with the stable release target
• prepare main for a clean v0.4.0 stable tag and stable Release workflow execution

Validation

• dotnet build --configuration Release --nologo
• dotnet test --configuration Release -v minimal --nologo
• dotnet pack -c Release src/Signal.CANdy.Core/Signal.CANdy.Core.fsproj -o artifacts-stablecheck
• dotnet pack -c Release src/Signal.CANdy/Signal.CANdy.fsproj -o artifacts-stablecheck

Notes

• this is a minimal follow-up PR after the already-merged integration PR release: integrate dev into main for 0.4.0-alpha.1 #15
• prerelease history remains documented in reports and the existing v0.4.0-alpha.1 tag; this PR only promotes the authoritative public version surface on main