feat: add encoding levels to RPC transport

[ashkalor]

Implements the encoding level architecture suggested by @kentonv in #133, handling serialization at the transport layer rather than through a global WireFormat hook.

Summary

• Add EncodingLevel type with four levels: stringify, devalue, partial, passthrough
• Add optional encodingLevel property to RpcTransport interface
• Add wrapTransport() helper for custom binary formats (CBOR, MessagePack, etc.)
• Serialize/deserialize based on transport's encoding level
• Maintain full backwards compatibility (stringify is the default)

Encoding Levels

Level	Format	Use Case
stringify	JSON string	Default, max compatibility
devalue	JS objects	Avoid double-parse overhead
partial	JS objects + raw Uint8Array	Binary data without base64
passthrough	Structured-clonable	postMessage, Workers

Motivation

Binary formats like CBOR can significantly outperform JSON for large payloads by avoiding base64 encoding cycles. This architecture lets transports declare their capabilities and have serialization handled appropriately.

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Latest commit: eddff04

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[burdiyan]

Having some examples in the README.md would be appreciated 🙏.

[ashkalor]

Having some examples in the README.md would be appreciated 🙏.

Hey just added them, let me know if it looks good. Didn't initially add them because I was expecting some kind of feedback.