feat(editor): animate the canvas in step with the simulation

A simulation store owns one run and one playhead, so the chart and the canvas
read off the same instant. While the panel is open, Stocks show a live value and
a fill gauge, Flows and Converters their current value; a play/pause + scrubber
drives the playhead (a wall-clock-paced clock in usePlayback), and a marker
tracks it on the chart.

src/composables/usePlayback.ts

@@ -0,0 +1,49 @@
+/**
+ * Playback clock — the wall-clock that advances the simulation playhead while
+ * `playing`. It lives in a composable (mounted once, in the Editor) because it
+ * needs `requestAnimationFrame` and component teardown; the simulation store
+ * stays pure state + actions.
+ *
+ * A run plays over a fixed wall-clock window regardless of how many samples it
+ * has, so a 40-step model and a 480-step one feel the same speed. Elapsed time is
+ * accumulated and drained in whole frames, so a slow tab catches up rather than
+ * drifting.
+ */
+import { onBeforeUnmount, watch } from "vue"
+import { useSimulationStore } from "@/store/simulation"
+
+/** Target wall-clock duration for a whole run, in milliseconds. */
+const RUN_DURATION_MS = 6000
+
+export function usePlayback(): void {
+  const sim = useSimulationStore()
+  let raf = 0
+  let last = 0
+  let carry = 0
+
+  function frame(now: number): void {
+    if (!sim.playing) return
+    carry += now - last
+    last = now
+    const msPerFrame = Math.max(16, RUN_DURATION_MS / Math.max(1, sim.frameCount))
+    while (carry >= msPerFrame && sim.playing) {
+      carry -= msPerFrame
+      sim.tick()
+    }
+    raf = requestAnimationFrame(frame)
+  }
+
+  watch(
+    () => sim.playing,
+    (playing) => {
+      cancelAnimationFrame(raf)
+      if (playing) {
+        last = performance.now()
+        carry = 0
+        raf = requestAnimationFrame(frame)
+      }
+    },
+  )
+
+  onBeforeUnmount(() => cancelAnimationFrame(raf))
+}