MCP documents Cargo support it already runs in production

The Model Context Protocol project documented Cargo as a supported package registry in PR #3315, months after the registry itself started accepting it. Cargo support went into the MCP registry with PR #1207. The package-types docs page never listed it, so Rust developers writing MCP servers had no documented way to publish through it.

The package-types page is meant to be the full reference for every registry type MCP's spec recognizes. A missing entry there acts like a missing feature to anyone reading it, even though Cargo already worked in production. Some Rust developers wrapped their servers in npm or PyPI packages instead, packaging systems built for JavaScript and Python. It got them a publish path they could point people to. The registry could already take registryType: "cargo" entries. The docs page just didn't say so.

The fix is a one-line docs edit, but it changes the default path for the next Rust MCP server. Anyone who already published an npm or PyPI wrapper as a substitute now maintains a wrapper that a native Cargo entry would have made unnecessary.