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/src/components/OriginGesture.vue

@@ -1,7 +1,6 @@
 <script setup lang="ts">
-import { computed } from "vue";
-import { useOriginGesture } from "../gesture";
-import type { OriginGestureProps } from "../types";
+import { useOriginGesture } from "../gesture/useOriginGesture";
+import type { OriginGestureProps } from "./types";
 
 defineOptions({ name: "OriginGesture", inheritAttrs: false });
 
@@ -12,7 +11,8 @@ const props = withDefaults(defineProps<OriginGestureProps>(), {
 /*
  * This convenience component makes the declaration live exactly where the
  * developer writes it. `useOriginGesture()` is also public for components that
- * cannot accept an extra wrapper element.
+ * cannot accept an extra wrapper element. Props are read once, at setup; the
+ * legacy `action` prop is read lazily so it may change.
  */
 const gesture = useOriginGesture(
   props.gesture ?? {
@@ -22,7 +22,6 @@ const gesture = useOriginGesture(
     action: (context) => props.action?.(context),
   },
 );
-const touchStyle = computed(() => gesture.style);
 </script>
 
 <template>
@@ -30,7 +29,7 @@ const touchStyle = computed(() => gesture.style);
     :is="as"
     v-bind="$attrs"
     class="nvo-gesture"
-    :style="touchStyle"
+    :style="gesture.style"
     @pointerdown="gesture.onPointerdown"
     @pointermove="gesture.onPointermove"
     @pointerup="gesture.onPointerup"