catalog: add CI assertion that build output is a single DLL

[CCoupel]

Context

Emby catalog requires that a plugin be distributed as a single `.dll` file. There is currently no CI check to enforce this constraint.

Problem

A dependency update or csproj change could silently introduce additional DLLs in the build output. If a PR is merged that breaks the single-DLL requirement, it will only be caught at catalog submission time.

Fix

Add a step in the GitHub Actions release workflow to assert exactly one `.dll` is present in `dist/`:

```yaml

• name: Assert single DLL output
  run: |
  count=$(find dist/ -name ".dll" | wc -l)
  if [ "$count" -ne 1 ]; then
  echo "ERROR: Expected 1 DLL in dist/, found $count"
  find dist/ -name ".dll"
  exit 1
  fi
  ```

Priority

Medium — Prevents silent regressions before catalog submission.