Phase 1: add native NOP request lifecycle

[bright2227]

Summary

• add a native Request type with prepared, submitted, completed, and teardown-cancelled states
• add Ring.prepare_nop(), Ring.submit(), and non-blocking batch Ring.reap()
• keep each request alive from SQE preparation until its CQE is consumed
• expose the lifecycle contract in the type stub and focused unit tests

Why

Phase 1 needs request ownership in native structs before real operations can safely move out of CFFI. A NOP is the smallest kernel operation that exercises the complete SQE-to-CQE lifetime without introducing buffer, socket, or cancellation complexity.

The ring owns a reference while a request address is visible to liburing. Reaping the CQE or closing the ring releases that ownership. Ring teardown marks outstanding requests cancelled with -ECANCELED.

Scope

This intentionally does not add blocking waits, asynchronous cancellation, multi-CQE requests, or operation buffers. Those remain separate reviewable milestones.

Validation

• native extension built against the vendored static liburing
• ruff check uringloop tests
• focused native lifecycle suite: 14 passed
• full suite: 31 passed
• Python development-mode stress run: 16,000 NOP requests completed
• git diff --check