Prepare 0.3.0-alpha.6 release

[Aaronontheweb]

Summary

• set the package version to 0.3.0-alpha.6
• publish the corrected stable-v0.3 consumer API migration notes from PR Correct the v0.3 consumer API before stable release #146
• record alpha.6 as the package Netclaw must validate before stable v0.3

Validation

• dotnet build -c Release --no-restore — 0 warnings, 0 errors
• dotnet test -c Release --no-restore — 2,870 passed
• dotnet pack -c Release --no-restore -o ./bin/nuget
• package metadata reports 0.3.0-alpha.6 for netstandard2.0 and net8.0
• release-workflow regex extracts the non-empty alpha.6 notes
• slopwatch analyze --no-baseline --fail-on warning — 0 issues
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• git diff --check
• adversarial release review: no content blockers

After this PR merges and Linux/Windows validation passes, push the bare 0.3.0-alpha.6 tag from the merge commit. The tag workflow will publish NuGet and symbol packages and create the GitHub prerelease.