Add go-widgets → PDF bridge (raster + vector) - #2

Summary

Adds a go-widgets → PDF bridge so any github.com/go-widgets/toolkit widget tree can be "printed"/exported to a PDF. A read-only dependency on toolkit + painter is added; go-widgets is not modified.

API

• (*Page) AddWidget(root toolkit.Widget, rect Rect, opts *WidgetOptions) error — raster. Lays the tree out at the target size, renders it through a painter.NewPixelPainter + root.Draw, and places the RGBA result as an image XObject at rect. Prints any UI exactly as it draws on screen.
• (*Page) AddWidgetVector(root toolkit.Widget, rect Rect, opts *WidgetOptions) error — vector. A painter.Painter (+ painter.Clipper) implementation that emits PDF vector operators (fills, strokes, rounded-corner Béziers, 1×1 pixels, clip) instead of pixels. Text drawn through the toolkit's built-in bitmap font reaches the painter's Text primitive and becomes real, selectable Type0 PDF text. Requires WidgetOptions.Font.
• WidgetOptions{Theme, Scale, Font} with defaults (DefaultLight, DefaultWidgetScale = 2).

Both levels achieved

Raster (must) and vector (stretch) are both implemented — the painter.Painter interface is only 7 methods + optional 2-method Clipper, small enough to implement cleanly, and the toolkit's bitmap font already delegates to p.Text for any non-PixelPainter back-end, which the vector painter turns into selectable PDF text.

Proof

Tests build a real widget tree (Container + prominent Button + Label), export to PDF, and re-open with the independent rsc.io/pdf reader as an oracle:

• raster: asserts the image XObject /Subtype /Image, its Width/Height, /Im0 Do in the content stream, and that decoded samples contain painted (non-black) pixels;
• vector: asserts a Type0 font resource plus text-show (Tj), fill (f), stroke (S), Bézier (c) and clip (W) operators in the content stream.

Gate

• Pure-Go, CGO_ENABLED=0, go 1.26.4.
• Exact 100% statement coverage.
• go vet clean, gofmt clean (new files).
• Builds on the 6 64-bit arches (amd64, arm64, riscv64, loong64, ppc64le, s390x) + js/wasm + darwin + windows.

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