The request asks for a clear, ordered list of how RustAPI will reduce external dependencies and what to replace first. This PR updates the docs to spell out the sequence, concrete actions, and candidate replacements.

• Roadmap (prioritized order)
  • Enumerate replacements in descending priority: routing → OpenAPI → TOON → templates, while keeping runtime/HTTP crates external.
• Execution plan
  • Document the steps to retire dependencies: internal adapters, contract tests, feature-flagged rollouts, and default flips.

1. Wrap external APIs behind RustAPI traits.
2. Add contract tests to freeze behavior.
3. Ship native implementations behind feature flags.
4. Flip defaults and deprecate external crates.

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