Restore Linux release artifact

[ianbruene]

Motivation

• Restore a missing downloadable Linux CD/release artifact so tagged releases include a Linux build alongside existing macOS and Windows artifacts.
• Keep the existing CI test matrix and Ubuntu mockgrbl smoke/integration steps unchanged while adding explicit Linux packaging to the CD flow.

Description

• Added a build-linux job to .github/workflows/cd.yml that installs Linux build deps, builds the GUI ddgo with -tags 'miqt serial', builds mockgrbl, packages them into dist/DDGo-linux-amd64.tar.gz, verifies the archive, and uploads it as the DDGo-linux-amd64 artifact with if-no-files-found: error.
• Updated the release job in .github/workflows/cd.yml to depend on the build-linux job, require the Linux tarball during tagged-release validation, and include it in the GitHub Release upload listing.
• Updated docs/RELEASING.md to document the new Linux artifact and the archive layout and build commands.
• Changed files: .github/workflows/cd.yml, docs/RELEASING.md.
• Artifact name and contents: uploaded artifact name DDGo-linux-amd64 containing DDGo-linux-amd64.tar.gz with linux-amd64/ddgo, linux-amd64/mockgrbl, and linux-amd64/README-artifact.txt.
• Build commands used for the artifact: GOOS=linux GOARCH=amd64 CGO_ENABLED=1 go build -tags 'miqt serial' -trimpath -ldflags='-s -w' -o dist/linux-amd64/ddgo ./cmd/ddgo, GOOS=linux GOARCH=amd64 CGO_ENABLED=1 go build -trimpath -ldflags='-s -w' -o dist/linux-amd64/mockgrbl ./cmd/mockgrbl, and tar -C dist -czf dist/DDGo-linux-amd64.tar.gz linux-amd64.

Testing

• Ran unit and package tests: go test ./... succeeded and go test -tags serial ./internal/transport ./internal/ports ./internal/app ./internal/grbl succeeded locally.
• Performed local build checks: go build -tags serial -o /tmp/ddgo ./cmd/ddgo and go build -o /tmp/mockgrbl ./cmd/mockgrbl succeeded and /tmp/mockgrbl -h printed usage successfully.
• Verified workflow YAML parse with ruby -e 'require "yaml"; YAML.load_file(".github/workflows/cd.yml")' succeeded and git diff --check reported no issues.
• Attempted a local miqt-tagged Linux build and dependency install (apt-get install qtbase5-dev) failed in this environment due to blocked package repositories, so full -tags 'miqt serial' Linux artifact build was not validated locally but the CD job installs qtbase5-dev on the runner; CI will perform the full Linux build on ubuntu-latest during tagged releases.

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