feat(engine): export LoadImages for non-raster painters

[tannevaled]

Summary

• `Engine.LoadImages` is a thin exported entry point over the existing (unexported) `loadImages`: fetch, decode, budget and viewport-fit every `` and inline ``, returning the intrinsic-size map `layout.LayoutDocument` needs and the bitmap map a painter needs, both keyed by element.
• Motivation: `go-pdfkit/html2pdf` runs the engine's own cascade + layout but paints to a vector PDF instead of the raster canvas. Without this it had no way to size or draw images short of re-implementing fetch/decode/budgeting — which the fleet doctrine says to share, not silo. `RenderDocument` uses the exact same code path, so the consumer draws images exactly as the engine would.
• No behaviour change for existing callers; `loadImages` itself is untouched.

Test plan

• `TestLoadImagesExported` — a data:-URI PNG round-trips offline into both maps at its 8x8 intrinsic size; a `display:none` and a src-less `` are excluded from both.
• `go test ./...` green across every package; `go vet ./...` clean; gofmt clean on the two touched files.

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