audio-knife: support deferred start params

[pragmatrix]

Summary

• Assemble opt-in deferred service parameters into a normal Start before ContextSwitch begins the conversation.
• Validate the immediate params message type, conversation ID, encoding, and inline/deferred exclusivity.
• Document the wire contract, client requirements, and protocol terminology.

Validation

• RUSTUP_TOOLCHAIN=stable cargo fmt --all
• RUSTUP_TOOLCHAIN=stable cargo test -p audio-knife

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Note

This pull request was developed with assistance from an AI coding agent alongside the author.