CI: `pack` job points to `src/` paths that do not exist

[beyondnetPeru]

The pack job in .github/workflows/build.yml lists every project under a src/ prefix:

- src/BeyondNetCode.Shell.Aop/BeyondNetCode.Shell.Aop.csproj
- src/BeyondNetCode.Shell.Aop.Aspects/BeyondNetCode.Shell.Aop.Aspects.csproj
# ...

There is no src/ directory in this repository — the project folders live at the repo root (BeyondNetCode.Shell.Aop/, BeyondNetCode.Shell.Aop.Aspects/, …).

Consequence: the pack job cannot succeed on a v* tag, so nothing is ever published. The v1.0.0 tag exists, and the NuGet profile currently shows 0 packages.

This is the highest-impact fix in the repo right now: it unblocks the whole publish chain (see also #2 and #3, which affect the same job).

Acceptance criteria

• The six paths in the pack matrix resolve to real .csproj files.
• dotnet pack <path> -c Release succeeds locally for each project in the matrix.
• A dry run on a test tag produces six .nupkg artifacts.

Where

.github/workflows/build.yml → pack job.