Registry of application-layer payload decoders.

New API (v0.5.0+)

Matchers can now return context to decoders for clean data flow:

DecoderRegistry.register(%{
  protocol: :my_protocol,
  matcher: fn layers, payload ->
    if my_protocol?(layers) do
      {:match, extract_context(layers)}  # Return context
    else
      false
    end
  end,
  decoder: fn context, payload ->  # Receive context
    decode(payload, context)
  end,
  fields: [...]
})

Benefits:

• ✅ No Process.put workarounds (thread-safe, no race conditions)
• ✅ More efficient (decode once in matcher, use cached result in decoder)
• ✅ Pure data flow (easier to test)

Legacy API (deprecated, will be removed in v1.0.0)

The old API is still supported for backward compatibility:

DecoderRegistry.register(%{
  protocol: :my_protocol,
  matcher: fn layers, payload -> my_protocol?(layers) end,  # Returns boolean
  decoder: fn payload -> decode(payload) end,  # Arity-1
  fields: [...]
})

Note: Deprecation warnings will be emitted when using the old API.

Decoder Entry Format

Each decoder entry supplies:

• :protocol — atom identifying the protocol (e.g., :http)

• :matcher — function returning false | {:match, context} when the decoder applies

• :decoder — function accepting (context, payload) and returning structured data
• :fields — optional list of field descriptors for extraction

Matchers receive the list of protocol layers returned by :pkt.decode/2 and the raw payload binary from the previous layer. Decoders receive the context from the matcher (or nil for old API) and the payload. Decoders should return {:ok, value} on success; any other return is wrapped in {:ok, value} automatically.