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

tsconfig.app.json

@@ -24,6 +24,7 @@
     "apps/demo/src/**/*.ts",
     "apps/demo/src/**/*.vue",
     "packages/*/src/**/*.ts",
-    "packages/*/src/**/*.vue"
+    "packages/*/src/**/*.vue",
+    "packages/*/tests/**/*.ts"
   ]
 }