Clear stale toolpaths on program replacement

web/worker/sim-session.mjs

@@ -1926,6 +1926,11 @@ export class SimSession {
       case "PROGRAM_LOAD": {
         if (["RUNNING", "PAUSED"].includes(this.task.state.state)) return finish(this.error(input, "task_state", "program_load_while_active"), "failed");
         const source = input.payload?.source ?? input.payload?.text ?? input.payload?.program_source;
+        // PROGRAM_LOAD is a replacement transaction. Clear the previous
+        // canonical preview before asking the WASM interpreter to parse the
+        // new source, so a parse/subroutine error cannot re-publish the old
+        // toolpath as if it belonged to the requested file.
+        this.previewState = null;
         if (this.xyzacProgram) {
           this.pendingXyzacMotion = null;
           this.pendingProgramControl = 0;