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/verify/README.md

@@ -15,6 +15,6 @@ ONLY=rewrite npx vitest bench --run --config packages/core-v2/verify/vitest.conf
 Run each implementation in its own process with `ONLY=`; running both in one
 process biases whichever runs second.
 
-Behavioural verification lives with the unit tests: `src/property.test.ts`
+Behavioural verification lives with the unit tests: `tests/property.test.ts`
 drives random operation sequences and pointer streams and checks the scene's
 structural invariants after every step. Set `PROPERTY_SEEDS` to run more.