registry.json index returns 404 — no way to discover what the registry publishes

Individual items resolve correctly (https://crashappsec.github.io/react-registry/r/theme.json, /r/button.json, /r/card.json, /r/badge.json all return 200), but https://crashappsec.github.io/react-registry/registry.json returns **404**.

Without the index there is no programmatic discovery: a consuming repo cannot enumerate available items, and tooling that lists a registry's contents has nothing to read. Found while adopting the registry in crashappsec/compass (its README names COMPASS as an in-scope consumer) — we had to probe item names by hand to confirm what exists.

Publishing the index alongside the per-item files at the documented path would fix it.

Start by reading the README for the documented registry layout and inspecting the existing per-item files such as /r/theme.json and /r/button.json. Add the missing registry.json at the documented path so https://crashappsec.github.io/react-registry/registry.json returns the published index and consumers can enumerate the available items.

Written by the indexing model from the issue text.