docs: add native-core uringloop roadmap

[bright2227]

Motivation

• Provide a single-source development roadmap that defines correctness, compatibility, and performance goals for a native-core implementation of uringloop and to guide phased work from Phase 0 (benchmarks & tests) through Phase 6 (productization).

Description

• Add ROADMAP.md describing the project goal, guiding principles, Phase 0–6 implementation plan, kernel feature matrix, productization notes, risks, and explicit non-goals.
• Document Phase 1–3 technical targets such as a native _uringcore extension, request lifetime/ownership model, provided-buffer pool, multishot handling, and native loop/transport migration guidance.
• Include Phase 0 requirements for a benchmark harness and a CPython-derived correctness test harness and the expectations for gated feature-probing and fallback behavior.

Testing

• Ran repository checks with git diff --check, which completed without reported problems.
• Verified the new document contains a single top-level title via rg -n '^# Roadmap:' ROADMAP.md.
• Confirmed repository status and recent history to ensure the change is present in the working tree and visible to review tools.

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Codex Task