v0.4.0

Features

• [breaking] Add Codec abstraction with JSON, MessagePack, and Raw codecs(efc1d5a)

Bug Fixes

• Mark Error::Codec inner as #source
• Surface peer close as WebsocketClosed in HandshakeContext::recv(cd67288)

Testing

• Add coverage for Codec trait and new HandshakeContext paths(aeb3c89)
• Push coverage to 98% lines / 96% regions(09d7364)
• Clarify raw-API test names and cover send_raw directly(b10d987)