refactor(dice): extract engine from md-commander

The dice engine lived under md-commander/hooks but is shared by :md-dice, spark tables, the journal /roll command, and dice inside variable expressions. Move it to src/components/dice so the md-commander widget can be retired without losing rolling.

Also promote the browser-console harness to a real jest suite; the file was named test.ts, so jest never ran it.

src/components/md-dice.tsx

@@ -1,6 +1,6 @@
 import { customElement, noShadowDOM } from "solid-element";
 import { createSignal, onMount } from "solid-js";
-import { rollFormula } from "./md-commander/hooks";
+import { rollFormula } from "./dice";
 
 export interface DiceProps {
   key?: string;