Improve protocol design: use &'static str instead of enums

[amitu]

Current Problem

Our current enum-based protocol approach has type safety issues:

enum StreamRequest {
    GetStreamInfo,
    RequestChunk { chunk_id: u64 },
}

enum StreamResponse {
    StreamInfo { ... },
    AudioChunk { ... },
}

Issue: StreamRequest::GetStreamInfo could accidentally return StreamResponse::AudioChunk - compiler won't prevent this.

Proposed Solution: &'static str Protocols

// Protocol constants - clean and efficient
pub const AUDIO_GET_INFO: &'static str = "audio.get_info";
pub const AUDIO_REQUEST_CHUNK: &'static str = "audio.request_chunk";

// Separate request/response types for each protocol
struct AudioInfoRequest;
struct AudioInfoResponse { total_chunks: u64, ... }

struct AudioChunkRequest { chunk_id: u64 }
struct AudioChunkResponse { data: Vec<u8>, ... }

Benefits

1. Natural constraints: Low cardinality enforced - protocols must be hardcoded constants
2. No runtime construction: Prevents dynamic protocol generation
3. Type safety: Each protocol has dedicated request/response types
4. Clean JSON: No enum variant names cluttering wire protocol
5. Easy scaling: Clean namespacing (audio., video., screen.*)

Implementation

Update fastn_p2p::client::call() and fastn_p2p::listen().handle_requests() to accept &'static str instead of enum types.

Priority

Medium - current enum approach works but this would be much cleaner for complex applications with many protocols.