Restore full Linux desktop packaging

[ianbruene]

Motivation

• The Linux release artifact previously shipped only raw binaries and a README which is insufficient to run the MIQT/Qt GUI on a typical Linux machine, so the build must deliver a staged distribution that includes runtime libraries and Qt plugins.
• The goal is to keep existing CI build behavior and mockgrbl integration unchanged while producing a self-checking runnable Linux distribution similar in spirit to the macOS and Windows artifacts.
• The workflow should fail early if required runtime libs/plugins are missing to avoid publishing broken artifacts.

Description

• Added scripts/package-linux-app.sh which accepts <ddgo-binary> <mockgrbl-binary> <output-dir>, recreates the output layout, copies ddgo/mockgrbl, discovers and copies ldd-resolved shared libraries into lib/, copies Qt plugin directories from qmake -query QT_INSTALL_PLUGINS (including platforms/libqxcb.so, xcbglintegrations, imageformats, styles, etc.), writes run-ddgo.sh, and writes README-artifact.txt while excluding core loader/glibc libs.
• Added scripts/verify-linux-dist.sh which asserts the staged layout (ddgo, mockgrbl, run-ddgo.sh, non-empty lib/, plugins/platforms/libqxcb.so, README-artifact.txt), runs ldd checks for not found on ddgo and plugins/platforms/libqxcb.so (both with the bundled LD_LIBRARY_PATH/QT_PLUGIN_PATH), runs mockgrbl -h, and prints the final layout.
• Updated .github/workflows/cd.yml build-linux job to install additional runtime packaging helpers (qtbase5-dev-tools, libxcb-*, file), build raw binaries into dist/build-linux, call scripts/package-linux-app.sh and scripts/verify-linux-dist.sh, then archive dist/DDGo-linux-amd64 as dist/DDGo-linux-amd64.tar.gz and upload artifact DDGo-linux-amd64.
• Updated docs/RELEASING.md to document the new artifact layout and usage, the launcher behavior (LD_LIBRARY_PATH, QT_PLUGIN_PATH, QT_QPA_PLATFORM_PLUGIN_PATH), and the compatibility caveat that the artifact is built on ubuntu-latest and is not guaranteed portable to every distro.

Changed files: .github/workflows/cd.yml, docs/RELEASING.md, scripts/package-linux-app.sh, scripts/verify-linux-dist.sh.
Artifact layout produced and documented:

DDGo-linux-amd64/
  ddgo
  mockgrbl
  run-ddgo.sh
  lib/
  plugins/
    platforms/
      libqxcb.so
  README-artifact.txt

The runtime launcher is run-ddgo.sh which sets LD_LIBRARY_PATH="$ROOT/lib", QT_PLUGIN_PATH="$ROOT/plugins", and QT_QPA_PLATFORM_PLUGIN_PATH="$ROOT/plugins/platforms" before exec-ing ddgo.
Bundling strategy: ldd on ddgo and on copied Qt plugin .so files to collect non-system shared libraries into lib/, copy Qt plugin directories discovered via qmake, and intentionally exclude the dynamic loader and core glibc/libs (e.g. ld-linux*, libc, libpthread, libm, libdl, librt).

Testing

• Script and YAML sanity checks passed: bash -n scripts/package-linux-app.sh scripts/verify-linux-dist.sh and the workflow YAML loaded successfully with a Ruby YAML parse check.
• Unit/integration test runs passed: go test ./... succeeded and go test -tags serial ./internal/transport ./internal/ports ./internal/app ./internal/grbl succeeded.
• Local build tooling checks passed: go build -o /tmp/mockgrbl ./cmd/mockgrbl succeeded and script files were made executable.
• Packaging verification that exercises the full Qt plugin copy and ldd-based bundling was skipped locally because qmake/qmake-qt5 is not installed in this environment, but the GitHub Actions build-linux job installs qtbase5-dev-tools so full packaging/verification will run in CI.

Remaining limitations

• The artifact is built on ubuntu-latest and bundles Qt/runtime libraries as provided by that runner, so compatibility with older or very different distributions is limited by the runner's glibc/system ABI and is not guaranteed portable to every Linux distro.
• No GUI-launch smoke test was added because there is no guaranteed safe non-GUI --help path; verification stops at ldd/layout checks and running mockgrbl -h.

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Codex Task