feat: add monochrome CAD plot color mode

[brownie-cake]

Summary

Implements the engine-level monochrome rendering policy requested by flyfish-dev/file-viewer#213.

• adds colorMode: 'source' | 'monochrome' and monochromeColor to CadViewer
• adds runtime setColorMode(), getColorMode() and getMonochromeColor() APIs
• applies one fixed render color through the shared entity/layer color resolver, covering Canvas2D, retained WebGL buffers, text overlays, fills and ByBlock inheritance
• preserves source documents, entity colors, geometry, line types, line weights, visibility and alpha
• rebuilds only the render scene when switching policy while retaining the active view
• forwards the policy to the native DWF/DWFx/XPS viewer when supported
• adds regression tests and API documentation

Native DWF dependency

Native DWF/DWFx/XPS color overriding is implemented in flyfish-dev/dwf-viewer#2.

File Viewer integration

The public File Viewer options and toolbar toggle are implemented in flyfish-dev/file-viewer#244, which closes flyfish-dev/file-viewer#213.

Validation

Latest head 393c33840c2e0ef2270e511af17f2b53f9c824b7 passes both repository CI workflows. The checks cover the complete library/demo build, regression tests—including source immutability, alpha preservation, ACI preservation and ByBlock alpha handling—and package dry-run.