Tests: add local DXF viewer regression coverage and build verification #3

Problem

The current package.json has build/start/dist scripts but no test script, and the repository has no GitHub Actions workflow under .github/workflows.

For a viewer that depends on DXF parsing/rendering, measurement state, Standard/Plus edition branching, recent-file/folder logic, and an Electron IPC boundary, relying only on manual launches makes regressions easy to ship.

Goal

Add a small, fast locally runnable automated test suite and build checks that validates the app's important non-visual behavior plus the renderer bundle/build.

First useful coverage

• path/recent-file/folder helpers
• Standard vs Plus feature gating
• DXF open/load state transitions
• unit conversion / ruler / measurement calculations
• guide snapping math where it can be isolated from WebGL
• IPC contract tests for main/preload (including the restricted-path work from the security issue)
• malformed/empty DXF handling
• a few known DXF fixture files from DXF Sketcher and external/common CAD sources

Local verification

GitHub Actions is disabled by maintainer decision (6 October 2026). Provide repeatable local commands for:

1. npm ci
2. the regression suite
3. npm run build:renderer
4. generated bundle freshness where the bundle remains tracked

Full Electron UI/WebGL screenshots do not need to be the first step. Start with deterministic unit/integration checks and include actual local results in PRs. Enabling hosted automation is outside scope.

Cross-project compatibility

Include a tiny fixture set exported by EriArk/-DXF-Sketcher so changes to either app do not silently break the normal Sketcher -> Viewer workflow. At minimum cover lines, arcs, circles, polylines, layers/colors and a mixed drawing.

Done criteria

• npm test (or equivalent) exists and runs locally.
• The tests and renderer build have documented local commands and results.
• A broken DXF load/measurement/edition/IPC behavior fails the local suite without launching the app manually.
• At least one DXF produced by DXF Sketcher is part of the regression fixtures.