Phase 1: make native ring EINTR-safe

[bright2227]

Summary

• retry native liburing submission and completion peeks after EINTR
• dispatch pending Python signal handlers before retrying
• reject a retry if a signal handler closed or reinitialized the ring

Why

Phase 1 requires explicit EINTR handling at the native prepare/submit/reap boundary. Without the completion retry, reap() could discard a partially accumulated completion batch when liburing returned -EINTR. Python signal dispatch also introduces reentrancy, so an internal generation guard prevents retrying against closed or replacement ring resources.

This is a stacked draft PR and depends on #19. Its base must remain feature/native-request-nop until #19 merges.

Validation

• vendored liburing native extension build
• uv run pytest -q tests/unit/test_uringcore_liburing.py — 15 passed
• uv run pytest -q — 32 passed
• uv run ruff check .
• git diff --check feature/native-request-nop
• codex review --base feature/native-request-nop — no actionable findings