Move core-v2 tests into packages/core-v2/tests

Tests no longer sit next to the implementation. They import the package
through ../src, the root tsconfig still type-checks them, and the
package build and declarations cover only src/.

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

packages/core-v2/README.md

@@ -308,6 +308,10 @@ Each module depends only on the ones listed above it.
 | `gesture/recognizer.ts` | Pointer-down admission, `DirectionalTracker`, `GestureRecognizer`, and `useOriginGesture`         |
 | `components/`           | `OriginScene`, `OriginGesture`, `OriginGestureSurface`, and the internal node host                |
 
+Tests live in `tests/`, outside `src/`. `tests/property.test.ts` checks the
+scene's structural invariants after every step of random operation sequences
+and pointer streams; set `PROPERTY_SEEDS` to run more than the default 60.
+
 Pure helpers used by a class (for example `matchesStart`, `stepSpring`, or
 `composeNodeStyle`) are module-level `export const`s that the class calls, so
 they can be tested directly without widening the class's interface.