Core seam for server-side and export embedders #37

Today

Everything renderable goes through drawShape, but its browser dependencies are hardcoded: the global Path2D class, OffscreenCanvas/document canvases for measurement and hatch tiles, and the browser Image loader with its warming cache. Any server-side renderer (see #36) must fork or monkey-patch globals to reuse the engine. The paint order (Editor.shapesSorted()) and several geometry/layout helpers are likewise unreachable behind the exports map.

Proposal

A small, dependency-free, behavior-preserving seam:

• setShapePlatform({ Path2D, measureCtx, tileCanvas }) — explicit backend injection (never global monkey-patching; the browser never calls it).
• drawShape accepts per-render opts.images (pre-decoded bitmaps, bypassing the loader), opts.patternCache and opts.Path2D — no cross-request shared state.
• Export the shared paint order as sortShapesForRender() (used by Editor.shapesSorted(), identical behavior), plus the textLayout / noteLayout / boundsUnion helpers embedders need.
• Full type declarations. No new dependencies, no DOM touched, full suite green.

Follow-up to #33 territory (font stacks) but independent of it; together they are everything an out-of-tree headless renderer imports from core.