Restructure core-v2 into layered modules

Rewrite the core-v2 internals without changing the public API or runtime
behavior. The four large files (types, scene, motion, gesture) become
layered modules with one responsibility each:

- shared/: geometry and environment helpers
- view/: originView recipes
- choreography/: effect types, presets, and composeEffects compositing
- actions/: forward/back/replace/above/under via one overload resolver
- scene/: SceneGraph storage, transitions (history rules), compositor
  (styles and interactivity), spring (pure stepper + rAF driver), and
  createOriginScene, which only wires them and owns async timing
- composables/: useOrigin and the node-scope injection key
- gesture/: builder, definition->policy normalization, start admission,
  DirectionalTracker, and a GestureRecognizer session state machine
  replacing fifteen loose closure variables
- components/: renderers and gesture hosts, plus their prop types

Verification (packages/core-v2/verify): differential fuzzing drives
main and this branch with identical random operation sequences and
pointer streams and requires identical scene state, styles, contexts,
callbacks, and promise results at every step. It caught every planted
mutation that applied. Benchmarks are at parity.

Also fix a pre-existing test that called the defineProps macro inside a
runtime defineComponent, and correct API.md's description of the flick
commit threshold to match the runtime.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Faarh2Ts6StJPcz2mRiHiR

packages/core-v2/verify/scene.bench.ts

@@ -0,0 +1,101 @@
+import { createApp, h, nextTick } from "vue";
+import { bench, describe } from "vitest";
+import * as next from "../src";
+import * as base from "./.baseline";
+import { component } from "./harness";
+
+type Impl = typeof next;
+const all = { baseline: base as unknown as Impl, rewrite: next };
+const impls = process.env.ONLY
+  ? { [process.env.ONLY]: all[process.env.ONLY as keyof typeof all] }
+  : all;
+
+const motion = next.defineOriginChoreography({
+  name: "bench",
+  effects: ({ progress }) => ({
+    frame: { transform: `translateY(${progress}px)` },
+    source: {
+      transform: `translateX(${-progress * 100}px)`,
+      opacity: 1 - progress,
+    },
+    target: {
+      transform: `translateX(${(1 - progress) * 100}px)`,
+      style: { filter: "none" },
+    },
+  }),
+});
+/** Long enough for both implementations to reach steady-state JIT. */
+const options = { time: 4000, warmupTime: 1500, warmupIterations: 2000 };
+
+const views = Array.from({ length: 12 }, (_, i) =>
+  next.originView(component(`V${i}`), undefined, { key: `v${i}` }),
+);
+
+/** A scene with `depth` simultaneously live, chained operations. */
+async function liveChain(M: Impl, depth: number) {
+  const scene = M.createOriginScene({ initial: views[0]! });
+  const handles: next.OriginOperationHandle[] = [];
+  let origin = scene.nodes.value[0]!.key;
+  for (let i = 1; i <= depth; i += 1) {
+    const handle = await scene.begin(origin, M.forward(views[i]!, motion));
+    handle.update(0.5);
+    handles.push(handle);
+    origin = handle.targetKey;
+  }
+  return { scene, handles };
+}
+
+for (const depth of [3, 10]) {
+  describe(`styleForNode, every node, ${depth} live chained operations`, async () => {
+    for (const [name, M] of Object.entries(impls)) {
+      const { scene, handles } = await liveChain(M, depth);
+      let p = 0;
+      bench(
+        name,
+        () => {
+          p = (p + 0.013) % 1;
+          handles[0]!.update(p);
+          for (const node of scene.nodes.value) scene.styleForNode(node.key);
+        },
+        options,
+      );
+    }
+  });
+}
+
+describe("push → back cycle (begin + finish, no animation)", () => {
+  for (const [name, M] of Object.entries(impls)) {
+    const scene = M.createOriginScene({ initial: views[0]! });
+    const root = scene.nodes.value[0]!.key;
+    bench(
+      name,
+      async () => {
+        const push = await scene.begin(root, M.forward(views[1]!, motion));
+        await push.finish({ commit: true, animate: false });
+        const pop = await scene.begin(push.targetKey, M.back(motion));
+        await pop.finish({ commit: true, animate: false });
+      },
+      options,
+    );
+  }
+});
+
+describe("rendered frame: update progress + Vue re-render, 6 live operations", async () => {
+  for (const [name, M] of Object.entries(impls)) {
+    const { scene, handles } = await liveChain(M, 6);
+    const host = document.createElement("div");
+    document.body.append(host);
+    createApp({ render: () => h(M.OriginScene, { scene }) }).mount(host);
+    await nextTick();
+    let p = 0;
+    bench(
+      name,
+      async () => {
+        p = (p + 0.013) % 1;
+        for (const handle of handles) handle.update(p);
+        await nextTick();
+      },
+      options,
+    );
+  }
+});