feat: add bpmnlint integration for BPMN diagrams

Surface model validation issues directly in rendered diagrams and in the
editor via bpmn-js-bpmnlint. A toggle button on the canvas shows clickable
e

feat: add bpmnlint integration for BPMN diagrams

Surface model validation issues directly in rendered diagrams and in the
editor via bpmn-js-bpmnlint. A toggle button on the canvas shows clickable
error/warning overlays on the offending elements.

- New `lint` attribute on <bpmnio>: on | off | inactive. The viewer defaults
to button-present-inactive; the editor defaults to active.
- Rules live in .bpmnlintrc (extends bpmnlint:recommended) and are packed into
the committed bundles at build time via bpmnlint-pack-config, since bpmnlint
cannot resolve rules in the browser. build:vendor handles this automatically.
- The lint module runs in a plain Viewer (every injected service is present;
editorActions resolves optionally), so no viewer upgrade was needed.
- Add syntax tests for the lint attribute; default output is unchanged when the
attribute is absent.

Refs #83

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