feat: add non-destructive monochrome color rendering

[brownie-cake]

Summary

Adds an engine-level fixed-color rendering mode for native DWF/DWFx/XPS previews.

• adds DwfViewerOptions.monochromeColor and LoadOptions.monochromeColor
• adds runtime setMonochromeColor() / getMonochromeColor() APIs
• maps W2D vector strokes, fills and text fallbacks without mutating parsed data
• maps XPS Fill, Stroke, SolidColorBrush and GradientStop colors while preserving brush alpha and resource references
• maps W3D mesh/material colors while retaining geometry buffers and material opacity
• leaves raster image pages, page background, UI chrome, geometry and line weights unchanged
• clears renderer caches when switching color policy so Canvas/WebGL/WASM paths stay consistent

Validation

• added scripts/validate-monochrome.mjs
• covers source immutability, implicit W2D colors, transparent brushes, XPS resources/gradients, W3D geometry sharing and opacity preservation
• latest head 1feff212f828f3761d69c781466fda3afdb64c3c passes both CI and Cloudflare Pages demo checks

Integration stack

This is the native-renderer prerequisite for:

1. feat: add monochrome CAD plot color mode cad-viewer#1 — shared Canvas2D/WebGL/text/entity color policy
2. feat: add CAD monochrome plot mode file-viewer#244 — File Viewer configuration and toolbar integration for [feature]: 希望增加黑白线条界面 file-viewer#213