chore: move more of the stream implementation from the root-level gorums package to the stream package

[meling]

Move Channel and Server Logic to internal/stream

The goal is to move channel.go and parts of server.go to internal/stream to co-locate related logic.

Proposed Changes

NEW: internal/stream/channel.go

• Move content of gorums/channel.go here.
• Rename channel -> Channel (Exported).
• Rename newChannel -> NewChannel (Exported).
• Rename request -> Request (Exported).
• Rename nodeClosedErr -> ErrNodeClosed (Exported).
• Rename streamDownErr -> ErrStreamDown (Exported).

NEW:response.go

• Move NodeResponse definition here.
• Move ErrTypeMismatch here (or to errors.go in stream).

NEW: marshaling.go

• Move unmarshalRequest -> UnmarshalRequest.
• Move unmarshalResponse -> UnmarshalResponse.

MOD: gorums/channel.go: DELETE the file.

MOD: node.go

• Update channel field in Node to *stream.Channel.
• Update newNode to call stream.NewChannel.
• Update NodeContext.enqueue to use stream.Request.

MOD: encoding.go

• Remove unmarshalRequest and unmarshalResponse.
• Update usages to stream.UnmarshalRequest / stream.UnmarshalResponse if needed (unlikely if they are only used by channel/server).

MOD: errors.go

• Remove ErrTypeMismatch.
• Add alias: var ErrTypeMismatch = stream.ErrTypeMismatch (or just re-export/define).

KEEP: server.go for now

• streamServer is harder to move due to Handler dependency.
• Option 1: Keep streamServer in gorums for now (Simpler).
• Option 2: Define stream.Handler interface and move streamServer to stream.Server.
  I recommend Option 1 for the first pass to ensure stability, then tackle server in a second pass.