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/tsconfig.json

@@ -9,6 +9,5 @@
     "outDir": "dist",
     "tsBuildInfoFile": "../../node_modules/.tmp/core-v2.tsbuildinfo"
   },
-  "include": ["src/**/*.ts", "src/**/*.vue"],
-  "exclude": ["src/**/*.test.ts"]
+  "include": ["src/**/*.ts", "src/**/*.vue"]
 }