Plan 1 of the web embedding SDK (TS-retirement Phase 2): a wasm Viewer that
parses a .op document, renders it read-only via CanvasKit by reusing
op-editor-ui's canvas_viewport, supports pan/zoom navigation, exposes
read-only JSON snapshots, and exports SVG. Type-gen reuses jian-ops-schema's
ts-rs export.
Additive: new crate plus surgical cold pub exposures (CanvasViewport::from_scene
in op-editor-ui; pub mod canvaskit + pub init_backend in op-host-web). No TS
deleted. 17 tests; wasm 2.2 MiB gzip (0 env.* imports); clippy -D warnings clean.
