RSN.8 Resin slicing on a worker thread

Built in the lab (2026-11-08). The resin slicer runs on a module worker (lib/printresin/rs-worker.js, started by rs-run.js) so the page stays alive while a big plate is sliced; the worker borrows the print stage's manifold file and ps-slice.js through the URLs the host already gives the filament slicer. The layers come back as run-length data, moved not copied. Cancel ends the worker. A page that cannot start a worker slices on the page as before. Same layers byte for byte as slicing directly (test-worker 14/0; the real-browser run confirms rs-worker.js starts). With FX.140 the drawing is also about 6 times faster. Not tried on a real printer.