Phase 1: promote the canonical _uringcore module

[bright2227]

Summary

• publish the selected static-liburing backend as uringloop._uringcore
• align the C initializer and native type identities with the canonical module name
• rename the type stub and focused tests, and verify the canonical wheel artifact
• update the Phase 1 decision/spike documentation to record the completed transition

Why

The accepted Phase 1 backend decision reserves _uringcore_liburing for the comparison spike and requires the selected implementation to move behind the production _uringcore name. This is the smallest remaining blocker before building additional native request behavior on a stable module boundary.

Impact

This intentionally removes the experimental _uringcore_liburing package artifact in favor of _uringcore. Request behavior and ABI version 2 are otherwise unchanged.

Validation

• native extension build: passed
• focused native tests: 15 passed
• full test suite: 32 passed
• Ruff: passed
• git diff --check: passed
• sdist and wheel build: passed
• Twine checks: passed
• installed-wheel dependency/import verification: passed
• wheel contains _uringcore and no _uringcore_liburing artifact
• codex review --base main: no actionable findings