Feature: generate a version-accurate Unity API index for lookup_api #40

Why it matters

A hand-curated list of 59 entries cannot compete with the model's own memory, so lookup_api adds little value today. What models genuinely lack is version-accurate facts: which members exist in Unity 6.x, which are obsolete, and what the exact signatures are. A generated index makes the tool authoritative instead of decorative, and fulfils the M2 roadmap goal of "500+ entries".

Proposal

• A build script that generates unity_api_index.json from Unity's ScriptReference (or from the UnityEngine/UnityEditor reference assemblies via reflection / System.Reflection.Metadata), capturing type, member, signature, [Obsolete] message, and since-version.
• Store per Unity version (6000.0 LTS, latest 6.x), selectable via a unity_version argument (defaulting to the version found by analyze_project).
• Keep the curated entries as "examples/notes" layered on top.
• Ranked fuzzy search; return obsolete status and replacement prominently.
• Run the generator in the weekly data workflow (as a PR), replacing the current lifecycle-only refresh.

Done when

lookup_api returns accurate signatures and obsolete warnings for any public UnityEngine type in the supported versions, with size kept reasonable (compressed or SQLite).