docs(README): add @modelcontextprotocol/fastify to middleware listing

[amandalal007]

Closes #2112.

Adds the published @modelcontextprotocol/fastify middleware package to the top-level README in three places:

1. Overview bullet (Express, Hono, Fastify, Node.js HTTP)
2. Middleware packages list
3. Installation commands

The package is already published and has its own packages/middleware/fastify/README.md, so this just documents the existing package.

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Latest commit: 5eec44d

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@modelcontextprotocol/client

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@modelcontextprotocol/codemod

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@modelcontextprotocol/express

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@modelcontextprotocol/fastify

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@modelcontextprotocol/hono

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@modelcontextprotocol/node

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commit: 5eec44d