Add a deferred mutation buffer to batch entity changes

Normalized profile

What this task asks for

Add a deferred command buffer that batches entity mutations during query iteration and flushes them at defined boundaries. Implement a deferred command buffer that batches entity mutations during query iteration. Add `world.deferred` providing `spawn`, `destroy`, `add`, `remove`, `addExclusive`, and `flush`. `addExclusive` replaces existing relation pairs with one and wildcard `'*'` clears all pairs. Execution triggers are `updateEach` exit, `flush`, or non-deferred mutation on an entity with pending commands. Entity `has` and `get` return the same results they would after flush. `autoDestroy` relations cascade respecting nullification.